GB2308338A

GB2308338A - Plasma etcher

Info

Publication number: GB2308338A
Application number: GB9526311A
Authority: GB
Inventors: Morna Baxter
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1995-12-22
Filing date: 1995-12-22
Publication date: 1997-06-25
Anticipated expiration: 2015-12-22
Also published as: GB9526311D0; GB2308338B

Abstract

A plasma etcher includes an etching chamber (1) in which a workpiece is to be etched communicating with a plasma chamber (3) in which the active etchant species are produced. A temperature control unit (8) located around the plasma chamber (3) keeps the chamber walls at a constant temperature and improves process control of the critical dimensions on the workpiece.

Description

PLASMA ETCHER Field ofthe Invention This invention relates to a plasma etcher and to a method of etching a workpiece in a plasma etcher.

Background of the Invention Plasma etchers are widely used in semiconductor manufacturing. In one particular type of etcher - known as a downstream etcher - an etching chamber in which a workpiece is to be processed is provided separate from, but communicating with, a plasma chamber in which a plasma may be excited in an atmosphere selected for its particular etching properties. In use, the active species of the plasma are drawn downstream into the etching chamber and across the workpiece in order to carry out the etching process.

Plasma etchers of the type described above have been found to exhibit run-torun variability in the minimum critical dimension, which is a measure of the minimum feature size obtainable with a particular etching system.

This invention seeks to provide a plasma etcher that delivers a more reliable etch and consequently to provide a method of etching a workpiece that enables a smaller minimum feature size to be produced.

Summarv of the Invention According to the present invention there is provided a plasma etcher comprising an etching chamber, a plasma chamber communicating with said etching chamber, a power source adapted for exciting a plasma in said plasma chamber, said plasma etcher further comprising temperature controlling means located around said plasma chamber for keeping the walls of the plasma chamber at a constant temperature.

In this way the run-to-run variation of the critical dimension for a given process is reduced, allowing more repeatable results to be obtained.

In addition, the present invention provides for a method of etching a workpiece comprising the steps of placing a workpiece in an etching chamber, producing a plasma in a plasma chamber communicating with said etching chamber by excitation of a gas in said plasma chamber, and drawing excited species in said plasma over said workpiece to etch said workpiece, said method further comprising the step of keeping the temperature of the plasma chamber walls substantially constant during said etching of said workpiece.

Bnef Description of the Drawings An exemplary embodiment of the invention will now be described with reference to the drawing in which: FIG.1 shows a conventional downstream plasma etcher FIG.2 shows a preferred embodiment of a plasma etcher in accordance with the invention.

Detailed Descrintion of a Preferred Embodiment Referring to FIG.1, there is shown a conventional downstream plasma etcher having an etching chamber 1 with a support 2 on which one or more workpieces (not shown) may be placed. Attached to and communicating with etching chamber 1 is a plasma chamber 3. During operation of the plasma etcher gas from gas source 5 is introduced into the plasma chamber 3, and energy is supplied by means 4 to the gas to initiate a plasma 6.

In the plasma etcher shown in FIG. 1 plasma chamber 3 takes the form of a quartz tube and energy is supplied to the plasma chamber 3 by a microwave source and waveguide. Other forms of plasma chamber and alternative methods of exciting a plasma will, however, be apparent to the skilled reader.

Energy transfer to the gas is maximised by the arrangement of microwave source, waveguide and quartz tube. Even so, some localised etching of the plasma chamber 3 is still found to occur in the region of plasma production.

Once a plasma 6 has been established in the plasma chamber 3, the excited species, or radicals, of the plasma are drawn downstream by a gas handling system 7 - typically a pump connected to the plasma etcher - into the etching chamber 1. The workpiece in the etching chamber 1 is arranged so that the radicals are drawn across the workpiece, resulting in a chemical interaction between the radicals and the workpiece and hence a chemical etch of the workpiece.

During repeated use of a plasma etcher of the type shown in FIG.1, a significant variation in the minimum critical dimension is found in successive runs even when all operating parameters are kept identical. In an illustrative etching step requiring the etching of polysilicon features with automatic endpoint detection and a given percentage overetch, a significant variation in the minimum feature size obtainable with the system is encountered.

The applicant has discovered that the variations in the minimum critical dimension and the minimum feature size are due to variations in the etch rate.

Analysis of this problem has shown that the starting temperature of the plasma chamber walls affects the rate of production of radicals in the plasma, which in turn influences the etch rate of the workpiece in the etching chamber.

In normal operation the variation in the etch rate is therefore dependent on the time interval between successive runs of the plasma etcher. An attempt to improve the control of the critical dimensions by decreasing the time between process runs produced little effect.

FIG.2 shows a plasma etcher corresponding to the preferred embodiment of the invention, whereby parts that are identical to the conventional plasma etcher of FIG. 1 are labelled with the same reference numerals.

As shown in FIG.2, in order to overcome the variability in etching rate caused by fluctuations in the temperature of the plasma chamber walls, a temperature control unit 8 is attached to the plasma chamber 3, enabling the plasma chamber walls to be fixed at a desired constant temperature during etching of a workpiece. It has been found to be particularly advantageous to employ a cooling unit attached to a sheath around the plasma chamber.

Optimal results are obtained when the cooling unit keeps the temperature of the plasma chamber walls at a constant 2-39C. Below this temperature, however, problems of condensation arise.

By setting the temperature of the plasma chamber walls in this manner, the reliability of the etching process obtained with the plasma etcher is improved.

Employing a plasma etcher of the type shown in FIG.2 for a polysilicon backetch process, the variation in the ratio of effective width to effective length WeffLff, which has been shown to be apparatus dependent, is reduced.

Claims

1. A plasma etcher comprising: an etching chamber (1) comprising a support (2) for at least one workpiece; a plasma chamber (3) communicating with said etching chamber (1); and a power source (4) adapted for exciting a plasma in said plasma chamber (3); characterised by temperature controlling means (8) located around said plasma chamber (3) for keeping the walls of the plasma chamber (3) at a predetermined temperature.

2. A plasma etcher according to claim 1, wherein: said plasma chamber (3) is a quartz tube.

3. A plasma etcher according to claim 1 or 2, wherein said temperature controlling means (8) are cooling means.

4. A plasma etcher according to any one of claims 1 to 3, wherein: said power source (4) is a microwave source provided externally to said plasma chamber (1).

5. A plasma etcher according to any one of claims 1 to 4, wherein: said plasma etcher is a downstream etcher comprising gas supply means (5) communicating with said plasma chamber (3) and gas flow control means (7) communicating with said etching chamber (1).

6. A method of etching a workpiece comprising the steps of: placing at least one workpiece in an etching chamber (1); and inducing a plasma (6) in a plasma chamber (3) communicating with said etching chamber (1) by excitation of a gas in said plasma chamber (3), excited species in said plasma (6) being drawn over said at least one workpiece to etch said at least one workpiece; characterised by keeping the temperature of the plasma chamber walls at a predetermined temperature during said etching of said at least one workpiece.

7. A method according to claim 6, wherein: said plasma chamber (3) is a quartz tube.

8. A method according to claim 6 or 7, wherein: said plasma (6) is excited by a microwave source provided externally to said plasma chamber (3).

9. A method according to any one of claims 6 to 8, wherein: said temperature controlling means (8) are cooling means.

10. A method according to claim 9, wherein said cooling means keep the temperature of said plasma chamber walls at 2-39C.

11. A plasma etcher substantially as hereinbefore described with reference to Figure 2.

12 A method of etching a workpiece substantially as hereinbefore described with reference to Figure 2.