PLASMA ETCHING APPARATUS

Abstract

A plasma etching apparatus includes first, second and third chambers, and a plasma generation device. An inner cross-sectional area and shape of the second chamber interior substantially corresponds to the upper surface of a substrate, and a substrate support is disposed so that, in use, the substrate is substantially in register with the interior of the second chamber, and the upper surface of the substrate is positioned at a distance of 80 mm or less from the interface between the second and third chambers.

Claims

1. A plasma etching apparatus for plasma etching a substrate, the apparatus including: a first chamber having a plasma generation region, the plasma generation region having a cross-sectional area and shape; a plasma generation device for generating a plasma in the plasma generation region; a second chamber into which the plasma generated in the plasma generation chamber can flow, wherein the second chamber defines an interior having a cross-sectional area and shape, and the cross-sectional area of the interior is greater than the cross-sectional area of the plasma generation region; a third chamber having a substrate support for supporting a substrate of the type having an upper surface to be plasma etched, wherein the third chamber has an interface with the second chamber so that the plasma, or one or more etchant species associated with the plasma, can flow from the second chamber to etch the substrate; in which: the inner cross-sectional area and shape of the second chamber interior substantially corresponds to the upper surface of the substrate; and the substrate support is disposed so that, in use, the substrate is substantially in register with the interior of the second chamber, and the upper surface of the substrate is positioned at a distance of 80 mm or less from the interface.

2. A plasma etching apparatus according to claim 1 in which the substrate to be etched and the interior of the second chamber each have at least one width, and the ratio of the width of the interior of the second chamber to the width of the substrate is in the range 1.15 to 0.85, preferably 1.1 to 0.9.

3. A plasma etching apparatus according to claim 2 in which the ratio of the width of the interior of the second chamber to the width of the substrate is in the range 1.5 to 1.0, preferably 1.1 to 1.0.

4. A plasma etching apparatus according claim 1 in which the substrate support is disposed so that, in use, the upper surface of the substrate is positioned at a distance of 60 mm or less from the interface.

5. A plasma etching apparatus according to claim 1 in which the substrate support is disposed so that, in use, the upper surface of the substrate is positioned at a distance of 10 mm or more from the interface.

6. A plasma etching apparatus according to claim 1 in which the ratio of the cross-sectional area of the plasma generation region to the cross-sectional area of the second chamber is in the range 0.07 to 0.7.

7. A plasma etching apparatus according to claim 1 in which the first and second chambers are co-axial.

8. A plasma etching apparatus according to claim 1 in which the first and second chambers are both of circular cross-section.

9. A plasma etching apparatus according to claim 1 in which the first chamber is of a bell jar shape.

10. A plasma etching apparatus according to claim 1 in which the interface is defined by a spacer element disposed between the second chamber and the third chamber.

11. A plasma etching apparatus according to claim 1 further including a baffle disposed on or near to the substrate support in order to channel a gas flow in the vicinity of the substrate.

12. A plasma etching apparatus according to claim 1 in which the substrate support is configured to support a substrate having a diameter of at least 200 mm.

13. A plasma etching apparatus according to claim 1 in combination with the substrate, the substrate being supported by the substrate support.

14. A kit for retrofitting an existing plasma etching apparatus in order to provide a retrofitted plasma etching apparatus according to claim 1, the kit including: an adapter for connection to one or more portions of the existing plasma etching apparatus, the adapter including a sleeve which is configured to act as the second chamber when the adapter is connected, and further including connection means permitting the adapter to be connected to said one or more portions of the existing plasma etching apparatus to locate the sleeve in place.

15. A kit according to claim 14 in which the adapter further includes one or more flange portions for connection to one or more portions of the existing plasma etching apparatus.

16. A kit according to claim 14 further including a spacer element configured to be disposed underneath the sleeve and connectable to the adapter and/or the existing plasma etching apparatus so as to define the interface between the second chamber and the third chamber in the retrofitted plasma etching apparatus.

17. A method of plasma etching a substrate including the steps of: i. providing a plasma etching apparatus according to claim 1; ii. causing the substrate to be supported by the substrate support so that the substrate is substantially in register with the interior of the second chamber and the upper surface of the substrate is positioned at a distance of 80 mm or less from the interface; iii. generating a plasma in the plasma generation region; and iv. causing the plasma, or one or more etchant species associated with the plasma, to etch the substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Embodiments of apparatus and methods in accordance with the invention will now be described with reference to the accompanying drawings, in which:

[0047] FIG. 1 shows silicon etch rate as a function of distance from a silicon wafer centre during etching using a conventional apparatus;

[0048] FIG. 2 is a cross-sectional view of an etching apparatus which has been retrofitted to provide a plasma etching apparatus of the invention;

[0049] FIG. 3 is a perspective view of the interface region between the second and third chambers of the plasma etching apparatus of FIG. 2;

[0050] FIG. 4 shows etch depth as a function of position on a 300 mm silicon wafer;

[0051] FIG. 5 shows silicon etch rate as a function of position on a 200 mm silicon wafer;

[0052] FIG. 6 shows silicon etch rate and etch depth uniformity as a function of the diameter of the second chamber when etching 200 mm silicon wafers; and

[0053] FIG. 7 shows silicon etch rate and etch depth uniformity as a function of the gap between the second chamber and the wafer upper surface when etching 200 mm diameter silicon wafers.

DETAILED DESCRIPTION OF EMBODIMENTS

[0054] FIG. 2 depicts plasma etching apparatus, shown generally at 10, of the invention. The embodiment shown in FIG. 2 is in fact a commercially available plasma etching apparatus which has been retrofitted to produce apparatus in accordance with the present invention. More specifically, the apparatus shown in FIG. 2 is a retrofit of plasma etching apparatus produced by the applicants and marketed under the trade name DSi. The apparatus 10 comprises a first chamber 12 in the form of a ceramic bell jar having a gas inlet 12a through which gases are introduced in order to produce plasma. A portion of the first chamber 12 is surrounded by an ICP source 14 which is used to initiate and sustain a plasma in at least a plasma generation region of the first chamber 12 in a manner which is well-known to those skilled in the art. The lower end of the first chamber 12 flares out into an intermediate portion of the apparatus 10. The intermediate portion is depicted generally at 16 in FIG. 2, and is the main retrofitted component in the apparatus 10. The intermediate portion 16 includes an adapter structure 18 having a sleeve 18a, upper flange portion 18b and lower flange portion 18c. The upper flange portion is connected to the first chamber 12 and other upper portions of the apparatus 10. The lower flange portion is connected to a third chamber 20. The sleeve 18a is sized to carry a reduced-diameter second chamber 22 in the form of a sleeve which is positioned and located within the sleeve 18a. The second chamber 22 can further comprise a lower ring 22a which in this embodiment is connected to the sleeve 22. Below the intermediate section including the second chamber there is the third chamber 20 which houses an electrostatic chuck (ESC) 24 for supporting a wafer 26 to be processed. The third chamber 20 includes a slot valve 20a for introducing the wafer 26 to the apparatus 10 and for removing same. The third chamber further includes an outlet 20b. Gases exit the apparatus from the outlet 20b using a suitable pumping arrangement (not shown) as is well-known to the skilled reader. It is noted that FIG. 2 does not show a complete view of the third chamber. Instead, FIG. 2 only shows an upper portion of the third chamber. The internal diameter of the third chamber 20 is of necessity considerably larger than the diameter of the wafer in order to enable the wafer to be introduced and removed from the apparatus 10. A cylindrical cover 28 is disposed around an upper portion of the apparatus 10 including the first and second chambers 12, 22 for safety purposes.

[0055] A baffle 28 is provided around the ESC 24 and a wafer 26 in order to increase the retention time of the etchant gas around the periphery of the wafer 26. A wafer edge protection (WEP) arrangement 30 is also provided.

[0056] In the conventional DSi apparatus, a different cylindrical structure serves as the second chamber, and its internal diameter is significantly greater than the diameter of the wafer. In the present invention, the internal diameter of the sleeve 22 (and the ring 22a) are matched to the diameter of the wafer to be processed. In a representative example, the wafer is a 200 mm diameter and the internal diameter of the sleeve 22 and ring 22a is also 200 mm. As described elsewhere herein, it is not mandatory that these diameters should correspond exactly, although advantageous results have been achieved with such an exact matching of the diameters. It will be appreciated that when the wafer 26 is mounted on the ESC 24, the wafer 26 is in register with the second chamber 22.

[0057] Examples of improved etching are now described using the apparatus shown in FIG. 2. Etching was performed in accordance with the Bosch process.

[0058] In FIG. 4 we show the improvements in process performance achieved in etch rate and uniformity for a Si etch process on 300 mm diameter wafers using a SF.sub.6 chemistry. By reducing the size (ID) of the second chamber from the standard 350 mm to 300 mm while maintaining a chamber to wafer gap of 43 and 23 mm, etch rates increase to 9.8 and 10.3 mm/min, respectively, while uniformity is also significantly improved over the standard value of 9.7%. The results are summarized in Table 1.

TABLE-US-00001 TABLE 1 Silicon ER (microns/min) and uniformity values for 300 mm silicon bulk wafer etched with a SF.sub.6 plasma; standard (350 mm ID) and reduced diameter (G = 300 mm ID). Etch rate Uniformity Gap [m/min] [%] G-23 mm 10.3 2.6 G-43 mm 9.8 7.4 Standard chamber 8.8 9.7

[0059] In FIGS. 5, 6 and 7 we can see representative results for 200 mm diameter wafers with a second chamber ID of 200 mm. A substantial improvement is seen in all cases when the 200 mm second ID chamber (with a 35 mm gap between the second chamber and the wafer) is compared with the standard 350 mm ID second chamber.

[0060] In FIG. 5 we can see a 15% improvement in etch rate for a Bosch Si etch process on patterned Si wafers between the standard chamber and the reduced diameter second chamber. Uniformity is also improved from +/9% with the standard chamber to +/6% with the smaller second chamber of the invention.

[0061] In FIG. 6 we can see the Si etch rate and uniformity for 200 mm diameter Si wafers as a function of the second chamber internal diameter with a fixed gap between the second chamber and the wafer of 35 mm. At 220-235 mm there is a large reduction in uniformity coupled with a more gradual decrease in etch rate as one moves towards larger second chamber IDS.

[0062] The importance of close coupling of the small lower chamber with the wafer is established in FIG. 7 where a Si etch process is conducted on 200 mm diameter wafers over a range (23-100 mm) of second chamber to wafer gaps. The optimum values for etch rate and uniformity are with the smallest gaps.

[0063] Without wishing to be bound by any particular theory or conjecture, it is believed that the advantageous properties described herein can be attributed to the combination of three factors. Firstly, the cross-sectional area of the interior of the second chamber is greater than the cross-sectional area of the first chamber, at least in the region where the plasma is generated. In this way, the volume in which the plasma is initially generated is not too large, and a relatively uniform initial plasma can be formed. In contrast, relatively large plasma generation chambers can give rise toroidally distributed plasmas. It is believed that if the initially generated plasma is not very uniform, then it is at best difficult to provide subsequent processing steps which result in uniform etching. Secondly, the diameter of the second chamber should be close to the diameter of the wafer. This is surprising, since it goes against the received wisdom in the art. In the unlikely but theoretical event that the wafer is not of circular cross-section, then the second chamber should be of a similar shape which closely matches the characteristic dimensions of the wafer. Thirdly, the gap between the wafer (in its in-use position during etching) and the closely matching second chamber should be small.

[0064] The apparatus provided by the invention can improve the gas and plasma containment above the plane of the wafer compared to prior art chambers of larger ID. The present invention can avoid or at least reduce the loss of etchant gas going directly to the pumping line, increase the etching rate, and/or improve the cross-wafer depth uniformity. Again, without wishing to be bound by any particular theory or conjecture, it is believed that the present invention can force the etchant gas to interact with the wafer around the wafer periphery before being pumped away. In practice, a balance should be found between this mixing and the reduced conductance that can be caused for pumping the etch products away from the wafer. A baffle might be provided around or in close proximity to the wafer to assist in this regard. However, the use of a baffle is not an essential feature of the invention. The skilled reader will realise that the invention can be implemented and optimised in many different ways, and such variations are within the scope of the invention. For example, it is not necessary that the wafer is supported by an ESC, or that a WEP arrangement is used. Also, instead of retrofitting an existing apparatus, it is possible to produce a new plasma etching apparatus in accordance with the invention. The third chamber may be pumped from a port located at the bottom of the chamber, instead of the side of the chamber. Other plasma generation devices might be contemplated.