GB2364434A

GB2364434A - Apparatus for anisotropic plasma etching

Info

Publication number: GB2364434A
Application number: GB0111847A
Authority: GB
Inventors: Klaus Breitschwerdt; Franz Laermer; Andrea Schilp
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2000-05-16
Filing date: 2001-05-15
Publication date: 2002-01-23
Anticipated expiration: 2021-05-15
Also published as: DE10023946A1; GB0111847D0; GB2364434B

Abstract

An apparatus for the anisotropic etching of a substrate (9) by means of a plasma (8) is proposed. To this end, with a plasma source (6) for generating a high-frequency electromagnetic alternating field in a reactor (2) a plasma (8) is generated from reactive particles by the fact that the high-frequency electromagnetic alternating field acts upon a reactive gas or a reactive gas mixture. In addition, a substrate electrode (10) is provided for accelerating an ion flow contained in the plasma (8) in the direction of the substrate (9), wherein between the plasma source (6) and the substrate (9) an aperture (14) formed as a perforated diaphragm is arranged. Said aperture (14) comprises a plurality of openings (32, 33 33') which are so arranged that the ions impinge at least virtually vertically out of the plasma (8) onto the whole of that surface of the substrate (9) which is exposed to the ion flow. This hopes to overcome the problems of beaking, profile tilting, profile curvatures and asymmetries.

Description

2364434 Apparatus for anisotropic plasma etching The invention relates to

an apparatus for the anisotropic etching of a substrate by means of a plasma according to the preamble of the main claim.

5 Prior art

There is known from patent DE 197 34 278 C1 an apparatus for the anisotropic etching of a substrate by means of a plasma, wherein a high-frequency electromagnetic alternating field is generated by means of an inductive plasma source and a reactive gas or reactive gas mixture is exposed to said alternating field in order to generate the plasma. Further it is provided there that the electrically charged particles generated are accelerated in the plasma is in the direction of the substrate, wherein an aperture is introduced between the plasma source and the substrate. By means of said aperture in the form of a preferably metallic diaphragm in combination with a superimposed cylinder there is achieved, in particular in combination with a plasma source after the manner of DE 199 00 179 Cl, a clear improvement in the uniformity of the etching rates achieved above the surface of the etched substrate. Further the use of said aperture with an additional cylindrical top piece leads to the minimising of a side wall undercut and etching profile curvature above the surface of the etched substrate, which is known in this connection as the so called "beaking" effect. To this extent there is achieved 5 by the aperture proposed in DE 197 34 278 C1, in addition to an improvement in uniformity, also a significant improvement in the profile shapes and the profile undercuts particularly in the substrate edge area.

10 On the other hand it has proved impossible in many cases, despite the clear improvements already achieved according to DE 197 34 278 C1, to suppress completely the "beaking" effect over the whole surface of the wafer to be etched, i.e. small but interfering profile differences are still is observed in the wafer edge area despite said aperture. In addition, there occurs through the ring-shaped outlet opening in the aperture introduced there a plasma expansion in the direction of the substrate, which leads to an electrical lens effect in the plasma and to an oblique 20 incidence of ions directed outwards, i.e. away from the wafer centre, onto the wafer surface. Consequently the side walls of etched structures are in some cases undercut non symmetrically and tilted slightly towards the wafer centre, i.e. a profile side wall that faces towards the wafer 25 centre exhibits a stronger profile tilt or undercut than a profile side wall that faces away from the wafer centre.

Although relatively small interference effects are involved here, the latter lead for example in the case of rotational 30 rate sensors to an amplification of a parasitic mode locking of the so-called fundamental oscillation mode into the so-called detection oscillation mode, which is described as the quadrature effect. It has been found that even very small profile asymmetries suffice to cause very large quadrature values, which affect the operating 5 efficiency of sensors of this kind considerably.

The object of the present invention was to improve the aperture construction known from DE 197 34 278 C1 in such a way that with retention or further improvement of the 10 uniformity of the etching rates..over the whole wafer surface the so- called "beaking" effect and the occurrence of asymmetries in the profiles produced is suppressed or eliminated. The aim of the present invention is therefore in particular to prevent the oblique deflection of the ions 15 during passage through the aperture opening as a result of the lens effect, in order in this way to ensure vertical profiles without profile tilting and asymmetries above all in the wafer edge area.

20 Advantages of the invention The apparatus according to the invention for the anisotropic etching of a substrate by means of a plasma has the advantage compared with the prior art that systematic

25 profile differences in particular at the edge of the substrate to be etched or side walls undercuts or etching profile curvatures are significantly reduced, so that at least virtually vertical profiles without profile tilting and asymmetries may be obtained over the whole surface of 30 the substrate. In addition, by means of the apparatus according to the invention the uniformity of the etching rates occurring over the whole surface of the substrate to be etched is retained or in some cases even improved beyond the uniformity already achieved according to the teaching of DE 197 34 278 C1.

Due to the fact that the aperture introduced now comprises a plurality of in particular circular openings, it is further obtained that the plasma below the aperture does not escape out of a single opening but out of a plurality 10 of openings in the direction of the substrate surface, wherein the expanding cones of the various aperture openings are superimposed, so that as a result the lens effect of a single opening in the aperture diaphragm is cancelled or at least substantially weakened. Overall, is therefore, the so-called "beaking" effect and the occurrence of profile tilts are at least largely eliminated. In particular it is now possible to etch anisotropic structures for example into silicon with extremely high etching rates, without inhomogeneities in 20 the plasma caused by the inductively coupled plasma source leading to profile bulges.

Advantageous developments of the invention follow from the measures given in the sub-claims.

Thus it is particularly advantageous if the aperture introduced is expanded laterally to an extent such that the outer aperture edge terminates flush with the plasma source, so that all gas intermixed with ions and radicals 30 has to escape out of the plasma source through the aperture with its plurality of openings in the direction of the I I substrate. It is thereby achieved that the position of the aperture relative to the substrate to be etched becomes non-critical, i.e. it is no longer necessary to place the aperture close to the substrate. Substantially greater 5 choice may therefore now be exercised regarding the distance of the aperture introduced from the substrate.

This is based on the fact that due to the flush termination, edge flows of the generated plasma that encompass the aperture no longer occur or reach the 10 substrate to be etched.

A further advantageous development of the invention provides selecting the arrangement of the openings in the aperture in such a way that the effective passage area from 15 the centre of the aperture to the outside decreases. In this way, thanks to the reduced passage areas, more intensive parts of the plasma in the outer area are shielded more effectively by the substrate to be etched than the less intensive plasma portions in the centre of 20 the aperture, which are faced with a large effective passage opening.

Further it is advantageous that the apparatus according to the invention now also makes it possible to dispense with 25 the cylindrical top piece known from DE 19 734 278 C1 in conjunction with the aperture, without this leading to inhomogeneities of the plasma or inhomogeneities of the etching rates over the whole surface of the substrate to be etched. Cost savings and a simplification of the 30 manufacture of the aperture thereby result. It should however be pointed out in this connection that the cylindrical top piece known from DE 197 34 278 Cl may also continue to be used on the aperture, i.e. the latter is therefore easily combinable.

5 All in all, therefore, the homogenisation of the ion flow to the substrate and the weakening of the ion flow in the outer areas may also be brought about with advantage merely through the geometry of the passage openings or hole sizes, as well as of the effective passage area.

Drawings The invention will be explained in greater detail from the drawings and in the following description. Figure la shows a top view onto a first embodiment of an aperture, Figure lb a section through Figure la, Figure 2a and Figure 2b show a second embodiment of the aperture, Figure 3a and Figure 3b a third embodiment of the aperture, Figure 4a and Figure 4b a fourth embodiment of the aperture, Figure Sa and Figure Sb a fifth embodiment of the aperture and Figure 6a and Figure 6b a sixth embodiment of the aperture. Figure 7 shows a diagrammatic representation of a plasma processing plant with an aperture according to the invention.

Embodiments The invention is based according to Figure 7 first of all on a plasma processing plant 1, such as has already become known in principle from DE 197 34 278 Cl. The plasma processing plant 1 comprises furthermore a reactor 2 into which is introduced via a feed nozzle 3 for example a fluorine-delivering reactive gas or a fluorine-delivering reactive gas mixture. The desired pressure in the reactor 2 may further be set wit1i a control valve 5 via a suction 5 nozzle 4. In addition a high-frequency plasma source is provided with an, ICP coil 6 for generating a high-density plasma 8, preferably according to the teaching of DE 199 00 179 C1. The coupling of the high- frequency magnetic field generated by the coil 6 into the reactor 2 loaded with

10 reactive gas leads to the ignition of the plasma 8. The substrate 9, which in the example explained is a 6-inch silicon wafer, is located on a substrate electrode 10 which is connected to a high-frequency voltage source 12.

15 In Figure 7 it is further shown diagrammatically that an aperture 14 is introduced between the inductive plasma source and the substrate 9. The aperture 14 consists for example of 2 mm to 15 mm thick aluminium. There is considered alternatively for the aperture 14, however, also 20 a different electrically conductive or in some cases also electrically isolating material such as a ceramic. The fixing of the aperture 14 in the plasma processing plant 1 takes place for example on a flange part (not shown). In addition it is provided in Figure 7 that above the aperture 25 14 is located a vertical cylinder 16 which consists for example of aluminium with a wall thickness of 10 mm. The cylindrical top piece 16 may further either be fixed to the aperture 14 or else be secured s eparately within the plasma processing plant 1. On the substrate electrode 3 is 30 installed further an absorber 17, which is coupled to the substrate electrode 10 in a thermally effective manner. The material of the absorber 17 is so selected that the individual reactive particles impinging there are absorbed and hence consumed. Silicon or graphite, for example, may be used in the present case for the absorption of fluorine particles.

As regards further known details on the plasma processing plant 1 according to Figure 7, further reference may be made, apart from the aperture 14'. described in detail below, 10 to DE 197 34 278 C1.

Figure la shows as a first embodiment the aperture 14 with a plurality of openings 30 in the form of circular holes each with a diameter of approx. 3 cm to 4 cm. In particular is it is provided according to Figure la that one opening 30 is located in the area of the centre point of the aperture 14, while the remaining four openings 30 are fitted concentrically and symmetrically around said centre point. The distance of the four outer openings 30 from the centre 20 point of the aperture 14 comes for example to 4 cm to 6 cm. Further the aperture 14 according to Figure la consists of aluminium with a thickness of typically 2 mm to 20 mm, preferably 5 mm to 10 mm. Figure lb shows a section through Figure la, where in addition the cylindrical top piece 16 25 is discernible, which is located on the side of the aperture 14 facing the plasma 8. Said cylindrical top piece 16 is in the explained example likewise made of aluminium and connected to the aperture 14. It has, adapted to a diameter of the substrate 9 of approx. 150 mm, a 30 diameter of typically 16 cm with a wall thickness of 5 mm to 20 mm. The height of the cylindrical top piece 16 comes furthermore to 20 mm to 80 mm.

Figure 2a explains a second embodiment of the aperture 14, 5 wherein by way of departure from Figures la or lb the cylindrical top piece 16 is now dispensed with. Further in Figure 2a the openings 30 are now concentrated more strongly in the area of the centre of the aperture 14, wherein their diameter comes for example to 2 cm. to 4 cm.

10 Figure 2b shows a section through Figure 2a.

Figure 3a shows a third embodiment of the invention, wherein there is now located in the centre of the aperture 14 an opening 3011 which has a diameter of 2 cm to 3 cm.

15 Around said opening 3011 are grouped in radial symmetrical arrangement four openings 301 each with a diameter of 4 cm to 5 cm. A particularly preferred performance of the invention further provides that by way of departure from Figure 3a the opening 301 ' in the centre of the aperture 14 20 is dispensed with, so that only four openings 301 arranged radially symmetrically around the centre point of the aperture 14 are present. In this case the openings 301 have a diameter of typically 4 cm to 5 cm and are located with a distance of their centre point from the centre point of the 25 aperture 14 of typically 4 cm to 6 cm with a diameter of the substrate 9 of approx. 150 mm.

Figures 4a and 4b show a further embodiment, wherein by way of departure from Figure 2a or Figure 2b a circular 30 opening 31 is now provided in the centre of the aperture 14, which has for example a diameter of 6 cm to 8 cm.

- 10 Around said opening 31 are arranged concentrically and symmetrically a total of 8 circular openings 31'. The openings 31' have a typical diameter of 2 cm to 4 cm.

5 It has proved to be particularly advantageous here if the decrease in the effective passage area of the sum of the openings 31' compared with the effective passage area of the opening 31 from the centre of the aperture 14 to the edge is approximately proportional to (x/r, where r is the 10 distance of the centre point of the openings from the aperture centre and a an empirically determined proportionality constant which depends on the geometry of the plasma source.

is Figures Sa and Sb show an aperture 14, with which there is placed around a central circular opening 32 with a diameter of approx. 3 cm an arrangement of circular slots, wherein the ring structure is interrupted by so-called retaining bars. Slots 32', 321 1 or ring segments are obtained in this 20 way, which are arranged concentrically around the centre point of the aperture 14. Further it is likewise provided here that the effective surface within a ring preferably decreases, like (x/r, from the inside to the outside. The width of the slot 32', 3211 according to Figure Sa or 5b 2S comes for example to 2 cm, wherein the decrease in the effective passage area is achieved in this case by the segmentation of the ring structures, i.e. the decreasing size of the retaining bars from the inside to the outside is obtained.

Figures 6a and 6b explain a further embodiment, wherein it is now provided to provide, in addition to a central, circular opening 32 in the centre of the aperture 14 with a diameter of typically 5 cm, further ring- shaped openings 33 5 and 331 in the form of ring segments which are arranged concentrically around the centre point of the aperture 14. Here also retaining bars are required in the area of the ring structures in order that the inner parts of the aperture remain fixed to the outer part and do not collapse 10 downwards. In this case the decrease in the effective passage area is ensured further by a decreasing width of the openings 33, 33' from the inside to the outside.

It should furthermore be emphasised that the embodiments 15 explained according to Figures 1 to 6 for the aperture 14 may also be combined with one another, by for example ring structures and hole structures being provided alternately from the inside to the outside.

20 The presence of in particular circular hole structures in the aperture has the advantage that a particularly effective prevention of the buildup of a lens effect due to the aperture 14 in the plasma is thereby achieved, since the plasma expansion cones below circular openings 25 counteract particularly effectively the build-up of an electrical lens effect in the plasma below the aperture 14 with the harmful effects explained.

It is furthermore now possible also to limit that aperture 30 area which is provided with openings to an inner area with a diameter of, for example, 7 cm to 10 cm. The passage area 4 ? or the area of the aperture 14 which comprises openings may therefore now, by way of departure from the teaching of DE 197 34 278 Cl, also possess a smaller outlet diameter than the diameter of the substrate 9 without profile tilts S occurring in the edge area because of lens effects in the plasma.

- 13

Claims

Claims

1. Apparatus for the anisotropic etching of a substrate by means of a plasma, with a plasma source for generating a high-frequency electromagnetic alternating field, a reactor for producing a plasma from reactive particles by the acting of the high frequency electromagnetic alternating field upon a reactive gas or a reactive gas mixture, and a substrate electrode for accelerating an ion flow contained in the plasma in the direction of the substrate, wherein between plasma source and. substrate an aperture formed as a perforated diaphragm is arranged, characterised in that the aperture (14) comprises a plurality of openings (30, 30', 30' ', 31, 311, 32, 32 1, 32 ' ', 33, 331) which are so arranged is that the ions impinge at least virtually vertically out of the plasma (8) onto that surface of the substrate (9) which is exposed to the ion flow.

2. Apparatus according to claim 1, characterised in that there is assigned to the aperture (14) at least one effective surface for the electrons/ions recombination, wherein the effective surface is formed as a roughly cylindrical top piece (16) on the aperture (14).

3. Apparatus according to claim 1 or 2, characterised in that the in particular circular aperture (14) comprises a plurality of circular openings (30, 301, - 14 301 1, 31, 31 1, 32) which are arranged in particular symmetrically around a centre point of the aperture (14).

5 4. Apparatus according to at least one of the preceding claims, characterised in that the centre point of the aperture (14) is occupied by a circular opening (30, 3011, 31, 32).

10 S. Apparatus according to at least one of the preceding claims, characterised in that the boundary of the aperture (14) terminates flush with the reactor (2) in such a way that the reactive gas intermixed with ions and/or radicals may escape out of the area of the is plasma source (6, 8) at least approximately exclusively through the aperture (14) in the direction of the substrate (9).

6. Apparatus according to at least one of the preceding 20 claims, characterised in that the openings (30, 301, 301 1, 31, 31 1, 32, 32 1, 32 ' 1, 33, 331) in the in particular circular aperture (14) are so arranged that the effective passage area of the aperture (14) which is occupied by the openings (31, 311, 32, 321, 3211, 2S 33, 33') decreases with increasing distance from the centre point of the aperture (14).

7. Apparatus according to at 1east one of the preceding claims, characterised in that the openings (30, 301, 30 301 1, 31, 31 1, 32, 32 1, 32 1 1, 33, 331) in the aperture have at least partly the form of slots(32 1, 321 1) and/or ring segments (33, 33') which are arranged in particular concentrically around the centre point of the aperture (14).

5 8.Apparatus according to claim 7, characterised in that the ring segments (33, 33 1) or the slots (32, 32 ') have a decreasing width with increasing distance from the centre point of the aperture and/or that the area occupied by retaining bars between the ring segments 10 (33, 33') or slots (32, 321 increases with increasing distance from the centre point of the aperture.

9. Apparatus substantially as hereinbefore described with 15 reference to Figures la, lb, and 7 of the accompanying drawings.

10. Apparatus substantially as hereinbefore described with reference to Figures 2a, 2b, and 7 of the accompanying 20 drawings.

11. Apparatus substantially as hereinbefore described with reference to Figures 3a, 3b, and 7 of the accompanying drawings.

12 Apparatus substantially as hereinbefore described with reference to Figures 4a, 4b, and 7 of the accompanying drawings.

13. Apparatus substantially as hereinbefore described with reference to Figures Sa, 5b, and 7 of the accompanying drawings.

14. Apparatus substantially as hereinbefore described with reference to Figures 6a, 6b, and 7 of the accompanying drawings.