DE20111766U1 - Retaining ring for wafer carriers for the chemical-mechanical polishing of semiconductor wafers - Google Patents

Description

Retaining ring for wafer carriers for chemical-mechanical polishing of semiconductor wafers
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The invention relates to a holding ring for a polishing device for the chemical-mechanical polishing of semiconductor wafers.

Chemical-mechanical polishing (CMP) is a planarization process used to level trench fillings, metal plugs, intermediate oxides and intermetallic dielectrics on semiconductor wafers or semiconductor wafers during the manufacture of semiconductor components. During chemical-mechanical polishing, the semiconductor wafer to be processed is pressed by a wafer carrier against a rotatable polishing table on which there is an elastic, perforated pad containing a polishing agent (slurry). The semiconductor wafer and the polishing table rotate in opposite directions, which polishes the surface of the semiconductor wafer at the protruding points until a certain level of flatness of the wafer surface is achieved.

In contrast to purely mechanical polishing, the polishing agent usually contains not only abrasive polishing grains, but also active chemical additives. The chemical additives, when matched to the layer material to be removed, allow the layers on the semiconductor wafer to be removed selectively.

The wafer carrier used in chemical-mechanical polishing essentially consists of three main parts, an upper part which is attached to a holding and rotating device, a wafer holder (wafer chuck) attached to the upper part, to the flat underside of which the semiconductor wafer is attached and held by means of negative pressure, and

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a retaining ring that surrounds the disk holder and is attached to it by means of an inwardly projecting flange. The retaining ring projects axially over the underside of the disk holder, but the projection is smaller than the thickness of the semiconductor wafer that is placed on the underside of the disk holder. The disk holder is attached to the upper part of the disk carrier via a ball joint in order to compensate for parallelism errors between the disk carrier and the polishing table.

During the polishing process, this disk carrier, to whose disk holder a semiconductor disk is attached, performs a rotary movement and a defined vertical pressure is exerted on the support of the polishing table. The retaining ring surrounding the disk holder counteracts the radial forces that act on the semiconductor disk during the polishing process, but does not touch the support of the polishing table. It also prevents the semiconductor disk from "slipping out".

The previous retaining rings used on the wafer carriers or wafer holders are complex, expensive parts due to the required dimensional accuracy and the material requirements. The material must not contaminate the semiconductor wafer with undesirable substances when it comes into contact with it. In addition, chemical resistance to the materials used in chemical-mechanical polishing (the pH value of these materials ranges from 1 to 12) is absolutely essential.

The retaining rings must be replaced regularly due to the considerable wear during polishing, which results in high maintenance costs. Reworking worn retaining rings is only possible to a very limited extent. Moreover, such a reworking is very complex, since two surfaces (the surface opposite the support of the polishing table and the flange contact surface on the disc

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holder) must be machined, the distance between which must be maintained very precisely and which must be exactly parallel.

The object of the invention is to design the retaining ring in such a way that the investment and maintenance costs of the polishing device are lower. Above all, the retaining ring should be designed in such a way that the service life is increased while maintaining high operational reliability.

This object is achieved by the retaining ring described in claim 1. Advantageous embodiments of the retaining ring are specified in the subclaims.

The retaining ring according to the invention consists of two parts, an upper part and a lower part. The upper part serves to hold and fasten the lower part. The lower part is a ring-shaped element which is preferably attached to the upper part from above with self-locking screws, for which purpose the upper part is provided with corresponding through-holes 0 or bores. The screws engage in threaded blind holes in the lower part.

Since wear essentially only occurs on the lower part, only this needs to be replaced. When reworking, only one surface (the surface opposite the polishing table support) needs to be machined. The correct projection for holding the semiconductor wafer can be set using shims between the upper and lower parts. If necessary, only the lower part is replaced. The material costs and production effort for the lower part are low, since a simple tube can be used as a semi-finished product to manufacture it.

The two-part retaining ring according to the invention can easily be exchanged for a conventional, one-part retaining ring. No changes to the disc holder are required. The changeover to the new, two-part retaining ring can therefore

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This can be done very easily, for example, as part of regular maintenance work.

The two-part retaining ring according to the invention generally has the advantage that the projection of the retaining ring required to hold the semiconductor wafer can be easily aligned with shims. The two parts of the retaining ring can advantageously be made from different materials, for example the upper part from high-alloy acid-resistant stainless steel and the lower part from a suitable plastic such as PEEK or PPS. The upper part of the retaining ring must also be resistant to the materials used in polishing, otherwise it will contaminate the polishing pad in the event of a chemical reaction and have a detrimental effect on the CMP process.

The invention is explained in more detail below with reference to the accompanying drawings. They show:

Fig. 1 shows schematically a polishing device for the chemical-mechanical polishing of a semiconductor wafer; Fig. 2 shows a conventional wafer carrier for the polishing device of Fig. 1;

Fig. 3 is an isometric sectional view of a two-part retaining ring for the disc carrier of the polishing device, together with the cover ring of the disc carrier, from above; and

Fig. 4 an isometric view of the two-part retaining ring from below.

In this description, the terms "top" and "bottom" or "upper part" and "lower part" always refer to the working position of a polishing device for the chemical-mechanical polishing process.

The polishing device 5 10 shown schematically in Fig. 1 has a rotatably arranged polishing table 12 on which there is an elastic, perforated support 14, the so-called pad, which is supplied via a feed 16 with polishing

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medium, the so-called slurry. A rotatable disk carrier 20 is arranged above the polishing table 12, which presses a semiconductor wafer 22 to be processed against the support 14 with a defined vertical pressure. The disk carrier 20 with the semiconductor wafer 22 attached to it and the polishing table 12 rotate in opposite directions during polishing. The polishing medium, which is fed via the feed 16, contains abrasive polishing grains and chemically active additives that enable selective removal of layers on the semiconductor wafer 22. Chemical-mechanical polishing is carried out primarily for planarizing trench fillings, metal plugs in contact holes and vias and intermediate oxides and intermetallic dielectrics.

Fig. 2 shows the disk carrier 20 in section. The disk carrier upper part 24 can be fastened to a holding and rotating device (not shown) which presses the disk carrier 20 vertically against the support 14 of the polishing table 12 with a defined force and in doing so sets the disk carrier 20 in rotation.

A disk holder 30 is attached to the disk carrier upper part 24 via a ball joint 26, and a semiconductor disk 22 can be placed on the flat underside 325 of the disk holder 30. The semiconductor disk 22 is held on the underside 32 by negative pressure. For this purpose, the disk holder 30 has holes 34 which are connected to a vacuum source (not shown).

In Fig. 2, the semiconductor wafer 22 is only indicated. The diameter of the wafer holder 30 corresponds exactly to the diameter of a semiconductor wafer 22, so that a semiconductor wafer 22 placed thereon essentially completely covers the underside of the wafer holder 30.

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The ball joint 26 between the disk carrier upper part 24 and the disk holder 28 can compensate for parallelism errors between the disk carrier upper part 24, which is firmly attached to the holding and rotating device, and the support 14 of the polishing table 12, so that the semiconductor disk 22 held by the disk holder 30 is pressed evenly and without tilting onto the support 14 of the polishing table 12.

The disc carrier 20 is covered by an upper cover ring 28, which protects the disc carrier 20 and in particular the ball joint 26 from the abrasive and chemically aggressive polishing agent.

The disk holder 30 is surrounded by a retaining ring 40. The retaining ring 40 is fastened to the disk holder 30 by means of a flange 42 projecting radially inwards on its upper edge, for example by means of screws 43 which extend through holes in the flange 42 and which are screwed into the body of the disk holder 30. It is also possible to fasten the retaining ring 40 to another element of the disk carrier 20, e.g. to the disk carrier upper part 24.

The upper cover ring 28 and the retaining ring 40 have approximately the same outer diameter. However, a gap or distance is provided in the axial direction between the upper cover ring 28 and the retaining ring 40 so that the disk holder 30 with the retaining ring 40 attached to it can carry out compensating movements around the ball joint 26 during the polishing process.

The retaining ring 40 projects downwards slightly beyond the underside 32 of the disk holder 30. The retaining ring 40 has the task of absorbing the radial forces acting on the semiconductor disk 22 placed on the disk holder 30 during the polishing process. On the other hand, however, the retaining ring 40 must not damage the support 14 of the polishing table 12.

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in order not to disturb the leveling of the surface of the semiconductor wafer 22 caused by the polishing process. The projection of the retaining ring 40 over the underside 32 of the wafer holder 30 is therefore large enough to hold a semiconductor wafer 22 placed on the underside 32 securely in the radial direction, but on the other hand the projection is smaller than the thickness of the semiconductor wafer 22 so that the retaining ring 40 does not rest on the support 14 of the polishing table 12.

In contrast to the case described at the beginning, the retaining ring 40 is not designed as a single piece, but as a two-part structure. The retaining ring 40 is divided axially into two parts by a horizontal cut.

Fig. 3 shows the retaining ring 40 together with the cover ring 28 of the disk carrier 20 obliquely from above in a cutaway view and Fig. 4 shows the retaining ring 40 alone obliquely from below.

The retaining ring 40 therefore consists of two parts, a retaining ring upper part 46 and a retaining ring lower part 48. The retaining ring upper part 46 has on its upper side the radially inwardly projecting flange 42 with the holes 44 for fastening to the disk holder 30. As can be seen in the sectional view of Fig. 3, the retaining ring upper part 46 is also provided in its outer ring section 52 with axial holes 54 for receiving screws (not shown) with which the retaining ring lower part 48 can be fastened to the retaining ring upper part 46. For this purpose, the screws engage in threaded blind holes 56 in the retaining ring lower part 48.

The screws used are advantageously self-locking so that they cannot loosen during the polishing process, which is accompanied by vibrations. Loose elements on the wafer carrier usually quickly lead to destruction of the semiconductor wafer 22 that has just been processed.

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The retaining ring lower part 48 otherwise has the shape of a simple ring without projections and the like, the radial dimensions (inner and outer diameter) of which correspond to the radial dimensions of the retaining ring upper part 46 (without flange 42). The axial length of the retaining ring lower part 48 is selected according to the respective requirements. It can, for example, be selected so that the axial lengths of the retaining ring upper part 46 and that of the retaining ring lower part 48 are approximately the same. The retaining ring lower part 48 can be manufactured easily and inexpensively, for example from a pipe section.

The two parts of the retaining ring 40 are made of a material that meets the requirements for mechanical and chemical resistance. The retaining ring upper part 46 and the retaining ring lower part 48 can be made of the same material, but it is advantageous for the upper part 46 and lower part 48 to be made of different materials in accordance with the different requirements.

The retaining ring upper part 46 can be made of high-alloy stainless steel such as X6CrNiMoTi 17-12-2, the surface of which is additionally tempered by electropolishing. The retaining ring lower part 48 can be made of the plastic material PEEK (polyetheretherketone) natural with a surface roughness Ra < 0.8 μ΄α .

It is also possible to manufacture the retaining ring upper part 46 from the plastic material PEEK or from the natural plastic material PPS (polyphenylene sulfide).

Since the retaining ring lower part 48 can come into contact with the semiconductor wafer 22 to be processed, when selecting the material for the lower part 48 it must of course be taken into account that contamination of the semiconductor wafer 22 must be avoided at all costs.

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Of course, the materials for the upper part of the retaining ring 46 and the lower part of the retaining ring 48 are also selected in such a way that chemical resistance to the substances used in the CMP process is guaranteed. 5

Since wear of the retaining ring 40 essentially only occurs on the lower part 48 during use, only this needs to be exchanged and replaced if necessary. A worn retaining ring lower part 48 can also be reworked by reworking the side surface of the retaining ring opposite the support 14 of the polishing table 12. This does reduce the axial length of the retaining ring lower part 48; however, the projection of the retaining ring over the underside 32 of the disk holder 30 can be relatively easily adjusted to the correct value using defined shims between the retaining ring upper part 46 and the retaining ring lower part 48. The retaining ring lower part 48 therefore does not always need to be replaced with a new part straight away, but can be reworked several times, for example to restore a defined low surface roughness. The service life of the retaining ring lower parts 48 and thus of the retaining ring 40 as a whole can thus be increased many times over.

But even if the length of the retaining ring lower part 48 has decreased to a uniaxial minimum dimension due to repeated machining, it can be replaced relatively inexpensively with a new lower part. The retaining ring upper part 46, which is much more complex to manufacture due to the flange 42 and is subject to considerably less wear, does not usually need to be replaced but can be reused.

Claims (4)

1. A retaining ring for a polishing device ( 10 ) for the CMP process in the formation of semiconductor elements on a semiconductor wafer ( 22 ), characterized in that the retaining ring ( 40 ) consists of an upper part ( 46 ) and a lower part ( 48 ), wherein the retaining ring lower part ( 48 ) is an annular element which is removably attached to the retaining ring upper part ( 46 ). 2. Retaining ring according to claim 1, characterized in that for fastening the lower part ( 48 ) to the upper part ( 46 ) by means of screws, the retaining ring upper part ( 46 ) is provided with through holes ( 54 ) and the retaining ring lower part ( 48 ) is provided with threaded blind holes ( 56 ). 3. Retaining ring according to claim 1, characterized in that for adjusting the projection of the retaining ring ( 40 ) over the underside ( 32 ) of the disc holder ( 30 ) one or more inserts are inserted between the retaining ring upper part ( 46 ) and the retaining ring lower part ( 48 ). 4. Retaining ring according to claim 1, characterized in that the retaining ring upper part ( 46 ) and the retaining ring lower part ( 48 ) consist of different materials.