GB2153305A - Polishing apparatus - Google Patents

Abstract

Chemical polishing apparatus comprises a ring (7) for mounting above a polishing pad (1). A holder (6) is mounted in liquid e.g. etchant, or gas e.g. air bearings (11, 12, 14) in the ring (7) and carries substrates (4) for polishing. Polishing etchant is supplied by a pipe (3). The pad (2) may be held stationary and is rotated on a table (1). A liquid e.g. etchant or gas e.g. air turbine (14, 15, 16) rotates the holder (6). Vertical positioning of the holder (6) in the ring (7) is determined by the rate of liquid or gas flow to the bearing (12). Prior to chemical polishing the substrate (4) may be mechanically polished in an abrasive slurry. <IMAGE>

Description

SPECIFICATION Chemical polishing apparatus The invention concerns apparatus for chemically polishing materials such as the semiconductors CdTe, GaAs, etc.

Crystallographic polishing apparatus usually entail the use of a freely rotating carrier upon which the crystal wafers to be polished are mounted. This carrier is placed on a polishing pad fixed to a rotating table, and a suitable abrasive polishing compound is applied to the crystal/pad interface. The polishing procedure can utilise several stages of succesively finer grades of polishing media and can produce surfaces free from imperfections as viewed by an optical microscope using Nomarski illumination. However, below surface damage is often still present after the final polish. This underlying damage can be exposed by acid eteching the polished surface, but cannot be removed without severe disruption of the surface topography.

Chemical polishing is similar to that of the above mechanical polishing and uses similar equipment with a polishing pad on the table.

However no abrasives are used. Instead an etchant solution is flooded over the flat polishing pad on the table. This solution chemically removes material from the substrate and can produce substantially undamaged surfaces but surface geometry is not maintained. Severe edge rounding and an orange peel surface appearance frequently occur. Details of polishing techniques are described in The Cutting 8 Polishing of Electro-Optic Materials" by G. W.

Fynn and W. J. A. Powell, published by Adam Hilger.

With the increasing demands for improved surface perfection on substrates intended for use in sub-micron planar technology, new polishing techniques are needed to ensure the elimination of this near-surface damage whilst maintaining the flatness required.

One new technique and variation on the above chemical polishing is described in Rev.

Sci. Instrum. 52(8) Aug 81 pp 1256-1259.

In this technique the two polishing surfaces are separated by the etchant. The etchant viscosity and the relative velocity between the substrate and table are arranged so that the substrate hydroplanes. This technique may be described as non-contact polishing. It has the advantage of producing flat surfaces without surface damage. Damage-free surfaces are required as the starting point for growing many semiconductor materials.

A difficulty of the above non-contact polishing technique is the dependence on etchant viscosity and rotational speed necessary to prevent contact of substrate and table.

According to the present invention the above problem is overcome by mounting a substrate holder on a fluid bearing so that the space between substrate and polishing pad are controlled by fluid pressure independently of etchant viscosity and rotational speeds.

According to this invention chemical polishing apparatus comprising a holder for carrying a substrate to be polished, and a mounting for locating the holder over a polishing pad is characterised by a fluid bearing arrangement for maintaining an adjustable space between substrate and polishing pad.

The fluid bearing may be arranged between the holder and mounting. Alternatively, or additionally, it may be between the holder and the polishing pad.

The holder may be freely rotatable in the mounting and located by a fluid bearing. The holder may be rotated in the mounting by a fluid turbine.

The fluid for the different bearings and turbine may be a gas such as air, or a liquid such as the polising etchant fluid and each may be independently controlled.

Substrates may be of CdTe, GaAs, etc. and other materials that can be chemically polished.

The polishing pad may itself be rotated independently of the holder, or may be held stationary.

The invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a plan view of chemical polishing apparatus; Figure 2 is a sectional side view of part of Fig. 1 and drawn to a different scale; Figure 3 is a side view of an alternative form of apparatus seen in Fig. 2; Figure 4 is a modification to Fig. 2.

As seen in Figs. 1, 2 apparatus for chemically polishing comprises a rotatable table 1 covered with a polishing cloth 2, e.g. a PEL LON (Trade Mark) pad or a polyurethane polishing material LP 57 (available from H. V.

Skan Ltd., and CERIUM Chemical Co. Ltd).

An etchant solution e.g. bromine methanol and glycerol is directed onto the table and pad 2 through a pipe 3. An electric motor (not shown) rotates the table 1 as required.

Substrates 4 to be polished are fixed on the underside of a mounting plate 5 itself secured to the base of a holder 6. This holder 6 is rotatable, about a vertical axis, inside a mounting ring 7 which is secured to a pillar 8 through an arm 9. A flange 17 is provided on the top of the holder 6 to position the holder 6 within the ring 7 and provide part of an air bearing. An air pipe 10 is connected to the ring 7 for supplying air to two air bearings; one a radial bearing 11, that allows the holder 6 to rotate and a second, an axial bearing 12, that supports the holder 6 and maintains a desired space 's' between substrate and pad.

A second air pipe 13 supplies air to a turbine 14 formed by indents 15 in the holder and jet nozzles 16 in the ring. Varying the air pres sure in the pipe 10 allows the substrate-pad gap S to be varied as the leaks, and hence pressure, of air between ring 7 and holder flange 17 varies. Alsok varying air pressure in the pipe 13 varies the energy supplied to the turbine and allows the rotational speed of the holder 6 to be varied.

In operation to polish substrates the substrates 4 are fixed to the plate 5 e.g. by a low melting point wax. The plate 5 is then attached to the holder 6 and the assembly placed in the ring 7. The arm 9 and pillar 8 are adjusted so that the axis of rotation of pad 2 and holder 6 are parallel and there is a required gap 's' between substrate 4 and pad 2 with an air supply to the radial and axial bearings 11, 12 Etchant fluid is then flooded onto the pad 2 and the pad 2 rotated. The air turbine 14 is caused to rotate the holder 6.

As an example, when polishing a substrate of CdTe the pad 2 of 20 cms diameter was rotated at 150 r.p.m., the holder 6 was rotated at 1,300 r.p.m., the substrate-pad gap was about 125um. Polishing for about 10 minutes removed about 10 um of material and produced a smooth mirror-like surface.

Prior to chemical polishing the substrate 4 may be mechanically polished in an abrasive slurry using a grit size of typically 1 um diamond for the final polish. Such mechanical polishing may be after the substrate 4 are fixed to the plate 5. After chemical polishing the substrates 4 are thoroughly washed with de-ionised water to remove all traces of etchant.

As an alternative to air bearings and turbine liquid bearings and turbines may be used. The liquid may be the same as the etchant liquid.

An alternative construction of apparatus is shown in Fig. 3 and is similar to that of Figs.

1, 2.

However, liquid is used for bearings and turbines. In addition ducts 20, in the holder 6 take liquid into the holder 6 and direct it downwards via a central bore 21 and radial ducts 22 into the gap s between substrate 4 and pad 2. Again varying the liquid supply varies the substrate-pad gap s. The liquid used is etchant.

In a further alternative (not shown) the bearings and turbine are air powered as in Figs. 1, 2. The holder is formed with a central bore with or without radially directed holes at its bottom. Etchant is poured into the bore of the holder and centrifuges out into the gap between substrate and pad when the holder is rotated.

In the modification shown in Fig. 4 the ring 7'is attached to the pillar 8 by a flexible arm 9'. The ring has an increased vertical height and rests on a fluid film 30 bearing on the pad 2. The fluid 30 may be air or the etchant supplied by the pipe 10 and channelled to the base of the ring 7'. Alternatively the ring may rest on excess etchant on the pad 2 supplied by the pipe 3. Channels (not shown) at the bottom of the ring 7' allow excape of excess etchant.

To polish the substrate 4 the ring 7' is first lowered onto the pad 2 without the holder 6.

The air bearing 11, 12 supply is then switched on and the holder 6 lowered into the ring 7'. Alternatively the air bearings 11, 12 supply may be switched on with the holder in place and then the ring 7' lowered onto the pad 2.

An advantage of resting the ring 7' on the pad 2 is the self alignment of ring 7 and pad 2.

Claims (8)

1. Chemical polishing apparatus comprising a ring for mounting above a polishing pad, a holder arranged for rotation within the ring and having a lower surface for carrying substrates to be polished, a fluid bearing for adjusting the relative vertical positioning between the ring and holder, and fluid driving means for rotating the holder within the ring.

2. The apparatus of claim 1 wherein the fluid driving means are shaped recesses in the holder and orifices in the ring forming a fluid turbine.

3. The apparatus of claim 1 wherein the fluid for the fluid bearing and fluid driving means is air.

4. The apparatus of claim 1 wherein the fluid for the fluid bearing and fluid driving means is etchant.

5. The apparatus of claim 1 and further comprising a table covered with a polishing pad, and means for mounting the ring about the polishing pad.

6. The apparatus of claim 5 and further comprising means for rotating the table independently of the holder rotation.

7. The apparatus of claim 5 wherein the ring is flexibly mounted over the table for location by the polishing pad.

8. The apparatus of claim 1 constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.