WO2006122578A1

WO2006122578A1 - Contaminant removal apparatus and method therefor

Info

Publication number: WO2006122578A1
Application number: PCT/EP2005/006635
Authority: WO
Inventors: Kevin Cooper; Scott Warrick
Original assignee: Freescale Semiconductor, Inc.
Priority date: 2005-05-17
Filing date: 2005-05-17
Publication date: 2006-11-23
Also published as: TW200707562A

Abstract

In the field of immersion lithography, it is known to provide a liquid (107) between an optical exposure system (104) and a wafer (100) carrying layers of photosensitive material to be irradiated with a pattern by the exposure system (104). However, particulate contaminants (124) are known to be suspended in the liquid (107), sometimes close to a surface (102) of the wafer (100) resulting in scattering of light emitted from the exposure system (104). The scattering causes the pattern recorded in the layers of photosensitive material to be corrupted, resulting in defective wafers. Therefore, the present invention provides a contaminant removal apparatus that employs a force generator, for example, an electrode (120) to generate an electrostatic force that can attract or repel the particulate contaminant (124) so as to remove the particulate contaminant (124) from a region between of the reservoir between the exposure system (104) and the wafer (100).

Description

CONTAMINANT REMOVAL APPARATUS AND METHOD THEREFOR

Field of the Invention

This invention relates to a contaminant removal apparatus of the type, for example, used to remove a particulate contaminant from a region within a reservoir in an immersion lithography apparatus. The present invention also relates to a method of removing a particulate contaminant from, for example, a region in a fluid-filled reservoir of an immersion lithography apparatus.

Background of the Invention

In the field of semiconductor processing, photolithography is a widely employed technique to "pattern", i.e. define a profile in one or more layer of semiconductor material, a semiconductor wafer. Using this technique, hundreds of Integrated Circuits (ICs) formed from an even larger number of transistors can be formed on a wafer of silicon. In this respect, for each wafer, the ICs are formed one at a time and on a layer- by-layer basis.

For about the last four decades, a photolithography apparatus, sometimes known as a cluster or photolithography tool, has been employed to carry out a photolithographic process. The cluster comprises a track unit that prepares the wafer, including providing layers of photosensitive material on the surface of the wafer prior to exposure to a patterned light source. To expose the wafer to the patterned light source, the wafer is

CONFiRMATIOM COPY transferred to an optics unit that is also part of the cluster. The patterned light source is generated by passing a beam of light through a chrome-covered mask, the chrome having been patterned with an image of a given layer of an IC to be formed, for example, transistor contacts. Thereafter, the wafer is returned to the track

^■unit for subsequent processing including development of the layers of photosensitive material mentioned above.

The wafer, carrying the layers of photosensitive material, is supported by a movable stage. A projection lens focuses the light passing through the mask to form an image on a first field over the layers of photosensitive material where an IC is to be formed, exposing the field of the layers of photosensitive material to the image and hence "recording" the pattern projected through the mask. The image is then projected on another field over the layers of photosensitive material where another IC is to be formed, this field over the layers of photosensitive material being exposed to the projected image, and hence pattern.

The above process is repeated for other fields where other ICs are to be formed. Thereafter, the wafer is, as mentioned above, returned to the track unit, and the exposed layers of photosensitive material, which become soluble through exposure depending upon the photosensitive materials used, are developed to leave a "photoresist" pattern corresponding to a negative (or positive) of the image of a layer of one or more ICs to be created. After development, the wafer undergoes various other processes, for example ion implantation, etching or deposition. The remaining layers of photosensitive material are then removed and fresh layers of photosensitive material are subsequently provided on the surface of the wafer depending upon particular application requirements for patterning another layer of the one or more ICs to be formed.

In relation to the patterning process, the resolution of the scanner impacts upon the width of wires and spaces therebetween that can be "printed", the resolution being dependent upon the wavelength of the light used and inversely proportional to a so-called "numerical aperture" of the scanner. Consequently, to be able to define very high levels of detail a short wavelength of light is required and/or a large numerical aperture.

The numerical aperture of the scanner is dependent upon the product of two parameters. A first parameter is the widest angle through which light passing through the lens can be focused on the wafer, and a second parameter is the refractive index of the medium through which the light passes when exposing the layers of photosensitive material on the wafer.

Indeed, to provide the increased resolution demanded by the semiconductor industry, it is known to reduce wavelengths of light used whilst also making lenses bigger to increase the numerical aperture. However, the limits to which the wavelengths of light used can be reduced are rapidly being reached, since wavelengths of less that 157 nm are absorbed by the lenses used.

Additionally, the above-described scanner operates in air, air having a refractive index of 1, resulting in the scanner having a numerical aperture between 0 and 1. Since the numerical aperture needs to be as large as possible, and the amount the wavelength of light can be reduced is limited, an improvement to the resolution of the scanner has been proposed that, other than by increasing the size of the lens, uses the scanner in conjunction with a medium having a refractive index greater than that of air, i.e. greater than 1. In this respect, the more recent photolithographic technique proposed, employing water and known as immersion lithography, can achieve higher levels of device integration than can be achieved by air-based photolithography techniques.

Therefore, scanners employing this improvement (immersion scanners) continue to use low wavelengths of light, but the water provides a refractive index of 1.4 between the lens and the wafer, thereby achieving increased resolution through increasing the numerical aperture of the immersion lithographic apparatus by a factor of 1.4.

Further, the refractive index of the water is very close to that of fused silica from which some lenses are formed, resulting in reduced refraction at the interface between the lens and the water. The reduced refraction allows the size of the lens to be increased, thereby increasing the numerical aperture further.

Whilst it appears that immersion lithography can achieve wafer throughputs comparable to air lithography, difficulties exist when introducing water between the lens and the wafer. One way of placing water between the lens and the wafer involves injecting a small film of water between the wafer and the lens, the film covering a field over the surface of the layers of photosensitive material where a given IC is to be formed, rather than the entire wafer.

However, by placing water between the lens and the wafer, and indeed in contact with the layers of photosensitive material, defects can be introduced. Such defects, or contaminants, when in the focal plane of the optics unit

(sometimes known as a "scanner") affect the ability of an immersion lithography apparatus to print defect-free lines and spaces. In this respect, defect levels can be affected by particle impurities in the water, temperature variations of the water, and thickness uniformity of the water layer. In relation to particle impurities, such impurities can originate from a number of sources and cause a number of process-related defects on the surface of the wafer and can, in some cases, prevent light reaching the surface of the layers of photosensitive material, thereby causing focussing problems. Also, the presence of contaminants can necessitate cleaning of a chamber between the optical system of the immersion lithography apparatus and the layers of photosensitive material. It is also possible that the particulate impurities will resettle on the layers of photosensitive material. Additionally, some lens elements exposed to the contaminants also require cleaning or replacement after a period of use. Clearly, such replacement of lenses is costly, thereby increasing production costs.

Statement of Invention

According to the present invention, there is provided a control apparatus and a method of regulating power as set forth in the appended claims. Brief Description of -the Drawings

At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a contaminant removal apparatus constituting an embodiment of the invention;

FIG. 2 is a schematic diagram of the apparatus of FIG-. 1 in use; and

FIG. 3 is a schematic diagram of a contaminant removal apparatus constituting a second embodiment of the invention.

Description of Preferred Embodiments

Throughout the following description identical reference numerals will be used to identify like parts.

Referring to FIG. 1, a semiconductor wafer 100 having layers of photosensitive material disposed thereon, the layers of photosensitive material having an upper surface 102, is disposed upon a substrate stage (not shown) of an immersion lithography apparatus. In this example, the immersion lithography apparatus is a modified TWINSCAN™ XT: 125Oi lithography scanner available from ASML. The lithography scanner is a complex apparatus having many parts, the structure and operation of which, are not directly relevant to the embodiments disclosed herein. Consequently, for the sake of clarity and conciseness of description, only the parts of the lithography scanner of particular relevance to the embodiments herein will be described.

The immersion lithography apparatus comprises an optical exposure (projection or catadioptric) system 104 connected to a liquid supply system 106, sometimes known as a "showerhead" . Liquid 107 is disposed between the bottom of the optical exposure system 104 and the surface 102 of the layers of photosensitive material.

The liquid supply system 106 comprises water inlet/outlet ports 108 in fluid communication with a reservoir 109 defined by an inner peripheral surface 110 of the liquid supply system 106 and the upper surface 102. A vacuum source (not shown) is coupled to vacuum ports 112, the vacuum ports 112 being in fluid communication with a first channel loop 114. A compressor (not shown) is coupled to air supply ports 116, the air supply ports 116 being in fluid communication with a second channel loop 118.

A first electrode 120, such as a copper electrode, is disposed at one side of the reservoir in a narrowed portion of the reservoir 109 formed between the optical exposure system 104 and the liquid supply system 106. A second electrode 121, for example also a copper electrode, is disposed at another side of the reservoir in the narrowed portion of the reservoir 109. The first and second electrodes 120, 121 are coupled to a driver circuit 122. In operation, particulate contaminants 124 are suspended in the liquid 107 in a region of the reservoir 109 between the optical exposure system 104 and the upper surface 102 of the layers of photosensitive material. The particulate contaminants 124 are, in this example, positively charged.

In order to remove the particulate contaminants 124 from the region of the reservoir 109, the driver circuit 122 is activated and generates a driver signal. In this example, the driver signal is a ^"'"continuous negative voltage signal, thereby causing the electrode 120 to exert an attractive electrostatic force on the particulate contaminants 124. The attractive electrostatic force urges the particulate contaminants 124 out of the region of the reservoir 109 and towards the electrode 120 (FIG. 2) .

Of course, the driver signal need not be a continuous negative voltage, and can be, for example, a continuous positive signal, thereby causing the particulate contaminants 124 to be repelled away from the electrode

120, but still out of the region of the reservoir 109.

Alternatively, the driver signal can be time-varying, for example a pulsed signal.

In another embodiment (FIG. 3), a filter 126, for example a micro tube or semi-permeable membrane filter , is disposed at an entrance to the narrow portion of the reservoir 109 to prevent the particulate contaminants 124 from reaching and adhering to the electrode 120. Consequently, when the electrode 120 is activated, the filter 126 acts as a barrier to the particulate contaminants 124. The pulsed signal mentioned in the previous embodiment is particularly, but not essentially, useful as pulsing of the electrostatic field generated by the electrode 126 serves to prevent clogging of the filter 126 with the particulate contaminants 124. Of course, the profile of the driver signal can be varied, for example, the driver signal can comprise an initial pulse followed by a continuous voltage signal.

Of course, it should be appreciated that in all of the above embodiments, the electrode 120 need not be disposed in the narrow portion of the reservoir 109, but instead at any location outside the region of the reservoir 109 that results in the particulate contaminants 124 congregating outside the region of the reservoir 109. Alternatively, the liquid supply system 106 can be provided with first and second protective channels (not shown) in which the first and second electrodes 120, 121 can be sited in order to prevent the contaminants reaching the first and/or second electrodes 120, 121. Consequently, any contaminants are attracted in the direction of the first and second electrodes 120, 121, but pass by mouths of the protective channels and continue towards the narrow portion of the reservoir 109.

Where the particulate contaminants 124 are not confined to accessing the electrode 120 via the entrance to the narrow portion of the reservoir 109, i.e. the electrode 120 is exposed from more than one direction, the filter 126 can be formed so as to surround the electrode 120.

Although reference to "scanners" (Step and Scan Systems) is made herein, the skilled person will appreciate that alternative optical exposure systems can be employed, for example a so-called "stepper" (step and repeat) in which a reticle passes between a light source and a lens system.

It is thus possible to provide a contaminant removal apparatus and a method of removing a particulate contaminant that permits immersion lithography to be a viable lithographic technique, reducing occurrences of defects and providing a wider range of photolithographic process parameters than available to existing photolithographic tools. Additionally, the immersion lithographic apparatus can operate for longer periods without the need to clean the optical exposure system. Consequently, higher yields of wafers can be produced.

Claims

Claims (PCT)

1. A contaminant removal apparatus for removing a particulate contaminant from a region within a reservoir in an immersion optical exposure system, the apparatus comprising: a drive signal generator (122) arranged to generate, when in use, a drive signal; and a force generator (120, 200, 202, 300) coupled to the drive signal generator (122); wherein the force generator (120, 200, 202, 300) is arranged, when in use, to apply a force for urging the particulate contaminant out of the region.

2. An apparatus as claimed in Claim 1, wherein the force generator (120, 200, 202) is arranged to generate a field.

3. An apparatus as claimed in Claim 2, wherein the field in an electric field.

4. An apparatus as claimed in any one of the preceding claim, wherein the drive signal is a time-varying signal.

5. An apparatus as claimed in Claim 4, wherein the time-varying signal is a pulsed signal.

6. An apparatus as claimed in any one of the preceding claims, further comprising a filter to act as a barrier to the force generator for preventing passage therepast of the particulate contaminant.

7. An apparatus as claimed in any one of the preceding claims, wherein the force generator is disposed in or at an entrance to a narrowed portion of the reservoir.

8. An apparatus as claimed in Claim 7, wherein the narrowed portion of the reservoir is between a projector system and a fluid supply system.

9. An apparatus as claimed in Claim 6, wherein the filter surrounds the force generator.

10. An apparatus as claimed in Claims 1 to 6, wherein the force generator is disposed in a protective conduit.

11. A method of removing a particulate contaminant from a region within a reservoir in an immersion optical exposure system, the method comprising the steps of: generating a drive signal; generating a force in response to the drive signal, thereby urging the particulate contaminant out of the region.