METHOD OF PREPARING A SILICON CARBIDE WAFER

Abstract

A method of preparing a silicon carbide wafer for subsequent epitaxial growth thereon is disclosed. The method comprises (a) placing the silicon carbide wafer onto a support table in a plasma processing chamber such that the surface of the silicon carbide wafer distal to the support table is unmasked; (b) establishing a flow of an etch gas mixture into the plasma processing chamber, wherein the etch gas mixture comprises molecular hydrogen, H.sub.2; and (c) generating a plasma from the etch gas mixture within the plasma processing chamber and using the plasma to etch the unmasked surface of the wafer so as to reduce the roughness of the unmasked surface.

Claims

1. A method of preparing a silicon carbide wafer for subsequent epitaxial growth thereon, the method comprising: (a) placing the silicon carbide wafer onto a support table in a plasma processing chamber such that the surface of the silicon carbide wafer distal to the support table is unmasked; (b) establishing a flow of an etch gas mixture into the plasma processing chamber, wherein the etch gas mixture comprises molecular hydrogen, H.sub.2; and (c) generating a plasma from the etch gas mixture within the plasma processing chamber and using the plasma to etch the unmasked surface of the wafer so as to reduce the roughness of the unmasked surface.

2. The method of claim 1, wherein the etch gas mixture further comprises a noble gas, wherein preferably the ratio of H.sub.2 to the noble gas in the etch gas mixture is in the range of 5:1 to 1.5:1, most preferably in the range of 4.5:1 to 2:1.

3. The method of claim 2, wherein the noble gas has an atomic weight greater than 28, wherein preferably the noble gas is argon, Ar.

4. The method of claim 2, wherein 5% to 40% of the etch gas mixture consists of the noble gas, preferably 10% to 35%, more preferably 15% to 30%.

5. The method of claim 1, wherein the etch gas mixture further comprises one or more fluorine-bearing gases each configured to release fluorine radicals when in the plasma, wherein preferably each fluorine-bearing gas is one of sulphur hexafluoride, SF.sub.6, carbon tetrafluoride, CF.sub.4, nitrogen trifluoride, NF.sub.3, and molecular fluorine, F.sub.2.

6. The method of claim 5, wherein the ratio of H.sub.2 to the fluorine bearing gas in the etch gas mixture is in the range of 100:1 to 5:1, preferably in the range of 50:1 to 20:1.

7. The method of claim 1, wherein at least 20% of the etch gas mixture consists of H.sub.2, preferably at least 40%, more preferably at least 50%, most preferably at least 60%.

8-12. (canceled)

13. A method of preparing a silicon carbide wafer for subsequent epitaxial growth thereon, the method comprising: (a) placing the silicon carbide wafer onto a support table in a plasma processing chamber such that the surface of the silicon carbide wafer distal to the support table is unmasked; (b) establishing a flow of an etch gas mixture into the plasma processing chamber, wherein the etch gas mixture comprises: one or more fluorine-bearing gases configured to release fluorine radicals when in a plasma; and one or more polymer-forming fluorocarbons and/or one or more polymer-forming hydrofluorocarbons; and (c) generating a plasma from the etch gas mixture within the plasma processing chamber and using the plasma to etch the unmasked surface of the wafer so as to reduce the roughness of the unmasked surface.

14. The method of claim 13, wherein the one or more fluorine-bearing gases comprise one or more of sulphur hexafluoride, SF.sub.6, carbon tetrafluoride, CF.sub.4, nitrogen trifluoride, NF.sub.3, and molecular fluorine, F.sub.2.

15. The method of claim 13, wherein the one or more polymer-forming fluorocarbons comprise octafluorocyclobutane, c-C.sub.4F.sub.8 and/or the one or more polymer-forming hydrofluorocarbons comprise trifluoromethane, CHF.sub.3.

16. (canceled)

17. The method of claim 13, wherein the ratio of the fluorine-bearing gas to the polymer-forming fluorocarbons and/or polymer-forming hydrofluorocarbons in the etch gas mixture is in the range of 15:1 to 3:1, preferably about 7:1.

18. The method of claim 13, wherein at least 90% of the etch gas mixture consists of the fluorine-bearing gas and the one or more polymer-forming fluorocarbons and/or polymer-forming hydrofluorocarbons, preferably at least 95%, more preferably substantially all of the etch gas mixture.

19-22. (canceled)

23. A method of preparing a silicon carbide wafer for subsequent epitaxial growth thereon, the method comprising: (a) placing the silicon carbide wafer onto a support table in a plasma processing chamber such that the surface of the silicon carbide wafer distal to the support table is unmasked; (b) establishing a flow of an etch gas mixture into the plasma processing chamber, wherein the etch gas mixture comprises at least hydrogen bromide, HBr and oxygen, O.sub.2; and (c) generating a plasma from the etch gas mixture within the plasma processing chamber and using the plasma to etch the unmasked surface of the wafer so as to reduce the roughness of the unmasked surface.

24. The method of claim 23, wherein: at least 40% of the etch gas mixture consists of HBr, preferably at least 50%, more preferably at least 60%; and/or the ratio of HBr:O.sub.2 in the etch gas mixture is in the range of 3:1 to 1.5:1, more preferably about 2:1.

25. (canceled)

26. The method of claim 23, wherein the etch gas mixture further comprises one or more halogen-bearing gases that do not comprise bromine.

27. The method of any of claim 26, wherein the one or more halogen-bearing gases comprise one or more fluorine-bearing gases each configured to release fluorine radicals when in the plasma, wherein preferably each fluorine-bearing gas is one of sulphur hexafluoride, SF.sub.6, carbon tetrafluoride, CF.sub.4, nitrogen trifluoride, NF.sub.3, and molecular fluorine, F.sub.2.

28. The method of claim 27, wherein the ratio of HBr to the fluorine bearing gas in the etch gas mixture is in the range of 100:1 to 2:1, preferably about in the range of 7:1 to 3:1.

29-42. (canceled)

43. The method of claim 1, wherein the wafer is oriented such that the unmasked surface is a silicon face.

44. (canceled)

45. The method of claim 1, wherein step (c) further comprises delivering helium to the surface of the wafer proximal to the substrate table so as to cool the wafer, wherein preferably the helium is delivered at a pressure in the range of 1 to 10 Torr, most preferably about 2 Torr.

46-50. (canceled)

51. The method of claim 1, wherein step (c) comprises: (c1) applying a bias voltage to the substrate table with a first power, P1, for a first time period; and (c2) after (c1), applying the bias voltage to the substrate table with a second power, P2, for a second time period, wherein P2 is less than P1, preferably less than 0.5 times P1.

52-54. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Examples of methods in accordance with aspects of the invention will now be described with reference to the accompanying drawings, in which:

[0057] FIG. 1 schematically depicts an exemplary surface processing tool adapted to carry out methods in accordance with the first, second and third aspects of the invention;

[0058] FIG. 2 shows an example of a process in accordance with embodiments of the invention; and

[0059] FIGS. 3(a)-(c) show illustrations of a silicon carbide wafer at different stages of preparation and subsequent epitaxial growth in accordance with embodiments of the invention.

DETAILED DESCRIPTION

[0060] FIG. 1 shows an example of a plasma processing tool suitable for implementing the presently disclosed methods. The plasma processing tool 1 comprises a plasma processing chamber 2 within which a silicon carbide (SiC) wafer 30 is placed during use. To perform etching, an etch gas mixture comprising one or more gases is introduced to the plasma processing chamber 2 and the conditions controlled in order to effect the desired etching mechanism. The process parameters within the chamber are controlled and can be adjusted by a set of at least one (but more typically a plurality of) devices, of which examples are shown schematically in FIG. 1. In this example, the tool 1 is equipped with two input gas supplies 4(a) and 4(b) for supplying first and second input gases, G1 and G2 respectively, to the plasma processing chamber 2. Some embodiments of the invention employ etch gas mixtures comprising more than two gases, and it will be understood that the plasma processing tool 1 may have any number of gas supplies each configured to supply a respective one of the component gases of the etch gas mixture. The ingress of each gas to the chamber 2 is controlled by valves 6(a) and 6(b) and respective mass flow controllers (not shown). Again, additional mass flow controllers may be provided where more than two gases are comprised by the etch gas mixture. The exhaust gas, including unreacted input gases and any reaction products, is removed from plasma processing chamber 2 via a duct 7 and associated pump(s) 8, the pump(s) 8 typically being capable of reducing the pressure within the chamber to near-vacuum conditions. The chamber pressure will be determined in the main part by the exhaust pump system and particularly the pumping speed and the “conductance” of the pumping line from the chamber to the pump (this is a factor related to the geometry of the pumping line). However during processing, when a plasma is created and/or when etching (or indeed deposition, as will be discussed later) takes place, gaseous species may be lost or created inside the chamber thereby having an effect on the pressure. In order to regulate for such variation, an automatic pressure control valve 8a is preferably provided as known in the art. The valve 8a changes the conductance of the pumping line to thereby enable the chamber pressure to be maintained substantially constant at the desired level as the plasma is struck and the material etched.

[0061] The plasma processing tool 1 is equipped with a plasma source for generating a plasma within the plasma processing chamber by means of an electrical discharge. Here, the plasma source is depicted as an inductively-coupled plasma source comprising a coil 9 surrounding the plasma processing chamber 2, which is supplied with RF power from power supply 10 via a RF matching unit 11. The RF matching unit 11 is configured to match the plasma impedance to that of the RF supply 10 in order to maximise efficiency of power transfer from the supply to the plasma. An example of a suitable matching unit is disclosed in WO-A-2010/073006. Other types of plasma source such as a capacitively-coupled plasma (CCP) or a microwave plasma source could be used instead.

[0062] The wafer 30 is mounted in use on a substrate table 14. As described below, a bias voltage is applied in use to the substrate 30 and this is achieved by connecting a voltage source 12 to the substrate table 14. If an RF power supply 12 is used then an Automatic impedance Matching Unit (AMU) may preferably be provided to ensure good coupling of power from the power supply 12 to the substrate table 14. The tool 1 may further comprise a temperature control unit 16 such as a heater and/or cooling system for adjusting the processing temperature of the substrate (additional devices for heating and/or cooling of the plasma processing chamber and plasma source may be provided to assist with process control and/or to maintain hardware stability). For instance, where etching is primarily to be carried out, the wafer 30 may be supplied with a coolant to prevent the significant amount of energy transferred to the substrate during ion bombardment and/or during exothermic chemical reactions causing an undesirable increase in the substrate temperature. The coolant could be helium, for example, which may be supplied to a surface 30b of the wafer 30 proximal to the substrate table 14. Preferably the helium is supplied to the surface 30b at a pressure of about 2 Torr.

[0063] As explained above, some embodiments of the invention involve growing an epitaxial layer on the surface 30a of the wafer 30 after the etching step. This step can be performed using a plasma processing tool 1 in which the gas supplies 4(a), 4(b) are configured to supply materials and/or precursors suitable for forming an epitaxial layer on the surface of the wafer. Of course, other methods of growing epitaxial layers could also be employed in these embodiments.

[0064] The devices operate upon instruction from a controller 20, such as a programmable logic controller (PLC) or similar. In some cases, more than one controller can be provided, with each controller controlling one or a subset of the devices. The controller is also connected to a user interface device such as a computer workstation 25 for receiving input from the user and/or returning outputs.

[0065] In FIG. 1, the data connections between the various devices and the controller 20 are indicated by dashed lines. In practice, this may be implemented as a network such as a CANbus bridge, which has connections to each of the devices as well as the user interface 25. The bus typically comprises multiple network channels including one or more data channels such as serial data channels (e.g. RS485) and, optionally, one or more power channels. The controller 20 issues commands across the bus, each of which is addressed to one or more of the devices and includes instructions as to one or more process parameters the device in question is to implement. An example of a network protocol which could be used for the issuing of commands for the control of the devices is given in WO-A-2010/100425. Of course, many other network implementations are possible as will be appreciated by the skilled person.

[0066] FIG. 2 illustrates a method of preparing an SiC wafer and performing subsequent epitaxial growth thereon in accordance with the first, second and third aspects of the invention. This method will be described with reference to the exemplary plasma processing tool 1 of FIG. 1, though it will be understood that the same method could be practised with other apparatus.

[0067] In step 201, an SiC wafer 30 is placed on the substrate table 14 inside the plasma processing chamber 2 as shown in FIG. 1. The SiC wafer 30 preferably consists of SiC only, though some impurities may be present, for example dopants. The SiC in the wafer 30 is preferably non-epitaxially grown, and may be grown from a melt, for example. The SiC wafer 30 is positioned on the substrate table 14 such the surface 30a distal to the substrate table 14 is unmasked, i.e. is not covered by a mask such as a patterned photoresist layer such that when a plasma is generated in the plasma processing chamber 2, substantially all of the surface 30a is exposed to the plasma. Usually the wafer 30 will be in the shape of a thin disc, having been obtained by slicing from a larger cylindrical ingot. Typically wafers produced in this way have diameters in the range of 100 to 200 millimetres (mm) and thicknesses in the range of 200 to 700 microns (μm). During step 201, the arithmetic average roughness of the unmasked surface 30a is preferably at least 1 nm and preferably also less than 100 nm, more preferably less than 10 nm. Roughness values in this range are typical of wafers that have been mechanically polished (but not chemically mechanically polished) as explained above.

[0068] Once the wafer 30 has been placed on the substrate table 14, a flow of an etch gas mixture into the chamber from the gas sources 4(a), 4(b) is established in step 203, and then a plasma is generated from this mixture at step 205 by powering the inductively coupled plasma source. Which gases are supplied in step 203 depends on which of the first, second and third aspects of the inventions is being performed, and detailed examples of etch gas mixtures suitable for implementing each aspect of the invention will be described later together with the other relevant process parameters. To illustrate this by way of a brief example, embodiments of methods in accordance with the second aspect of the invention could involve supplying a fluorine-bearing gas such as SF.sub.6 from one gas supply 4(a) and a polymer-forming hydrofluorocarbon such as CHF.sub.3 from a second gas supply 4(b) during step 203.

[0069] For as long as the plasma is sustained by the plasma source, it etches the unmasked surface 30a of the wafer 30. Since the surface 30a is unmasked, the etching occurs in a substantially uniform manner across the unmasked surface 30a and may thus be described as a blanket etch. The plasma may be sustained by the plasma source for a predetermined period of time, typically in the range of about 10 to 20 minutes. During this time period, the features of the plasma processing tool described above may be used to control parameters such as the substrate table bias power, the plasma power supplied by the coil 9, the pressure inside the plasma processing chamber 2, and the temperature of the wafer 30. Optionally, the substrate table 14 may be biased with a voltage while the plasma is present in the chamber. Hence, in step 207 (which is optional, as signified by dashed lines) a bias is applied to the substrate table 14 with a first power P1 while the plasma source continues to power the plasma. It may be that the same power P1 is maintained for the duration of the etch, but in some preferred implementations, as noted above, the bias power is reduced for a final portion of the etch. Hence, the method may optionally involve reducing the bias power during the optional step 209 to a value that is lower than P1, preferably less than half of P1. For example, typically P1 is in the range of 400 to 1000 W while P2 is in the range of 50 to 200 W. The reduced bias power P2 is preferably sustained for period of time in the range of 1 to 5 minutes, while the total duration of the etch is in the range of 10 to 20 minutes as mentioned above.

[0070] Once the etch has been performed for the required amount of time, the plasma may be extinguished in step 211. This can be achieved by evacuating the etch gas mixture from the plasma processing chamber 2 and/or turning off the supply of power to the coil 9 of the plasma source. Optionally, an epitaxial layer 213 may then be grown on the etched surface in step 213. As noted above, this may be performed by transferring the prepared wafer 30 to a chamber for epitaxial growth, which could be a separate chamber provided as part of the same plasma processing tool 1 by which the etch was performed or part of a separate device altogether. It should be noted that this step is not necessarily performed by the same entity and/or at the same apparatus, facility or site as the preceding steps. For example, the wafer 30 may be prepared in accordance with steps 201-211 by a manufacturer of wafers and then shipped to customers who perform the epitaxial growth step 213. Since methods in accordance with the present invention are capable of reducing the surface roughness of an SiC wafer 30 to a value suitable for performing epitaxial growth, there is no need to perform a chemical mechanical polishing step between steps 211 and step 213 (or indeed at any other time). The epitaxial layer be a layer of SiC or any other material of which an epitaxial layer can be formed on an SiC surface. The process could involve forming additional epitaxial layers of the same and/or different materials, which could then be etched to produce devices such as transistors and diodes.

[0071] FIGS. 3(a) to (c) show cross-sectional views of an SiC wafer 30 at various stages of its preparation in accordance with the present invention. FIG. 3(a) shows the wafer 30 prior to etching, for example during step 201 of the method of FIG. 2. The surface 30a that is to be prepared for epitaxial growth thereon can be seen to have an undulating profile with elevations and depressions (though these are much exaggerated and not to scale with the thickness of the wafer 30), which may be the result of damage caused by the slicing, lapping and polishing of the wafer 30 and/or the presence of defects at the surface 30a. The dashed line in this drawing shows the arithmetic average roughness R.sub.a of the surface 30a, which is the average deviation of the actual surface profile from a nominal plane of the surface, which is indicated by the solid line. R.sub.a is typically in the range of 1 to 100 nm, preferably 1 to 10 nm, before the wafer is subjected to etching in accordance with the present invention. Furthermore, in some embodiments R.sub.a may be at least 5 nm before the etch is performed. For simplicity, the profile of the surface 30b has not been illustrated here, though it will be understood that this surface will typically have a similar roughness provided it has undergone the same processing as the surface 30a. Preferably the surface 30a that is to be etched is an Si face, i.e. a face that terminates in a layer of Si atoms. This can be achieved simply by ensuring that the plane of the wafer 30 is suitably oriented with respect to the crystallographic axes of the crystal lattice as the crystal is manufactured.

[0072] FIG. 3(b) shows the wafer 30 after being etched during a method in accordance with the present invention, for example during or after step 211 of the exemplary method of FIG. 2. It can be seen the surface roughness R.sub.a′ of the etched surface 30a′ is considerably less than the initial value R.sub.a. As noted above, R.sub.a′ is preferably less than 3 nm, more preferably less than 1 nm and still more preferably less than 0.5 nm.

[0073] FIG. 3(c) shows the wafer 30 after an epitaxial layer 31 has been grown on the etched surface 30a′, as is performed in some embodiments of the invention, for example during step 213 in the process of FIG. 2.

[0074] We will now present experimental results showing the reductions in surface roughness of SiC wafers achieved by methods in accordance with embodiments of each of the first, second and third aspects of the invention together with the process parameters by which these results were achieved. We will refer to these as processes 1, 2 and 3 respectively. In each process, steps 201, 203 and 205 of the process of FIG. 2 were performed using an SiC wafer 30 for which the arithmetic average roughness of an unmasked surface 30a was measured before and after being etched. The SiC wafers 30 that were used were produced by Physical Vapour Transport and had diameters in the range of 100 to 150 nm, and the surfaces 30a that were etched included both Si faces and C faces (i.e. faces terminated in Si and C atoms respectively).

[0075] The experimental parameters and the results that were obtained are shown in the pairs of tables 1(a) and 1(b), 2(a) and 2(b), and 3(a) and 3(b). Each table lists the flow rates at which the gases in the etch gas mixture were introduced to the plasma processing chamber 2 in standard cubic centimetres per minutes (sccm), the pressure of the plasma inside the plasma processing chamber 2, the power supplied to the coil 9 of the plasma source, the power of the bias applied to the substrate table 14 and the duration of the etch for each process are listed in these tables together with the measured values of the arithmetic average roughness of the unmasked surface 30a of the before the etch in step 201, R.sub.a, and the arithmetic average roughness of the same surface after the etch at step 211, R.sub.a′. In each of processes 1, 2 and 3, helium was supplied to the surface 30b of the wafer 30 proximal to the substrate table (in a backside cooling configuration) at a pressure of 2 Torr so as to ensure that the wafer 30 and substrate table 14 did not heat to undesirably high temperatures during the etch. For the exemplary processes 1, 2 and 3, it has been found that the values of the pressure inside the plasma processing chamber 2, the power supplied to the coil 9, the power of the bias applied to the substrate table and the pressure of the helium can each be varied by at least ±20% of the values stated here while still successfully preparing the surface 30a of the SiC wafer 30 for epitaxial growth.

[0076] Tables 1(a) and 1(b) shows the experimental parameters listed above and the results obtained for two experiments, experiments number 1 and 2, which were each performed in accordance with embodiments of the first aspect of the invention. In one of these processes, an etch gas mixture composed of only H.sub.2 and Ar was used, while in the other two, the etch gas mixtures were composed of H.sub.2, SF.sub.6 and Ar. As noted above, the first aspect of the invention only requires the presence of H.sub.2 in the etch gas mixture, though a fluorine-bearing gas such as SF.sub.6 and a noble gas such as Ar are both preferred additives.

TABLE-US-00001 TABLE 1(a) Plasma Experiment H.sub.2 flow rate SF.sub.6 flow rate Ar flow rate pressure no. (sccm) (sccm) (sccm) (mTorr) 1 150 0 50 20 2 200 5 50 5

TABLE-US-00002 TABLE 1(b) Plasma Substrate Etch Experiment source table bias duration R.sub.a R.sub.a′ no. power (W) power (W) (minutes) (nm) (nm) 1 3000 400 20 1.13 0.70 2 2500 1000 20 1.1 0.41

[0077] Tables 2(a) and 2(b) show the experimental parameters listed above and the results obtained for two experiments, experiments number 3 and 4, which were performed in accordance with embodiments of the second aspect of the invention. The etch gas mixtures used in these processes were composed of CHF.sub.3 and SF.sub.6. In these exemplary processes, only one hydrofluorocarbon and one fluorine-bearing gas were present in the etch gas mixture. However, any number of other fluorocarbons and/or hydrofluorocarbons could be provided as alternatives or in addition to the CHF.sub.3 that was chosen for this implementation. Other fluorine-bearing gasses besides SF.sub.6 could also be utilised.

TABLE-US-00003 TABLE 2(a) Plasma Experiment CHF.sub.3 flow rate SF.sub.6 flow rate pressure no. (sccm) (sccm) (mTorr) 3 10 70 3 4 10 70 3

TABLE-US-00004 TABLE 2(b) Plasma Substrate Etch Experiment source table bias duration R.sub.a R.sub.a′ no. power (W) power (W) (minutes) (nm) (nm) 3 3200 800 10 6.22 2.44 4 3200 1000 10 1.3 0.8

[0078] Tables 3(a) and 3(b) show the experimental parameters listed above and the results obtained for three experiments, experiments 5 and 6, which were performed in accordance with embodiments of the third aspect of the invention. The etch gas mixtures used in these processes were composed of HBr, O.sub.2 and SF.sub.6. As noted above, the only essential components of the etch gas mixture for performing the third aspect of the invention are HBr and O.sub.2, though the presence of a fluorine bearing-gas such as SF.sub.6 is preferred.

TABLE-US-00005 TABLE 3(a) Plasma Experiment HBr flow rate O.sub.2 flow rate SF.sub.6 flow rate pressure no. (sccm) (sccm) (sccm) (mTorr) 5 60 40 10 15 6 80 20 20 20

TABLE-US-00006 TABLE 3(b) Plasma Substrate Etch Experiment source table bias duration R.sub.a R.sub.a′ no. power (W) power (W) (minutes) (nm) (nm) 5 3000 400 10 8.50 1.54 6 2500 200 10 4.76 0.76

[0079] It can be seen that each of the exemplary processes used to conduct experiments 1-6 reduced the surface roughness of the etched surface 30a to values suitable for epitaxial growth and hence successfully prepared the surface 30a for subsequent epitaxial growth thereon.