BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to photolithographic illumination systems. More particularly, this invention relates to cleaning a reticle prior to its use in a photolithography tool.

2. Related Art

Photolithography (also called microlithography or optical lithography) is a semiconductor device fabrication technology. Photolithography (hereinafter referred to as lithography) uses ultraviolet or visible light to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as diodes and transistors, can be fabricated as integrated circuits using lithographic techniques. Exposure systems or tools are used to implement lithographic techniques, such as etching, in semiconductor fabrication. An exposure system typically includes an illumination system, a reticle (also called a mask) containing a circuit pattern, a projection system, and a wafer alignment stage for aligning a photosensitive resist covered semiconductor wafer. The illumination system illuminates the reticle with a preferably rectangular slot illumination field to produce an image of the reticle circuit pattern. The projection system projects the image of the reticle circuit pattern onto the wafer.

The illumination system of a lithographic tool includes a light source. Excimer lasers are one such light source and operate at several characteristic wavelengths ranging from vacuum ultraviolet light to greater than 400 nanometers (nm) depending on the gas mixture used, as represented below.

Wavelengths for excimer lasers

Excimer

Wavelength

XeF

351 nm

XeCl

308 nm

KrF

248 nm

KrCl

222 nm

ArF

193 nm

F

2

157 nm

By shortening the wavelength of the light, the resolution of the optical projection system is improved. Thus, in a lithography tool it is desirable to utilize a light source with wavelengths within the vacuum ultraviolet range, i.e., below 200 nm.

As shorter wavelength light sources are used in lithography, organic contamination in the exposure area of the lithography tool becomes a greater problem. It is well known that organic contaminates have high optical absorption coefficients at shorter wavelengths, particularly at 157 nm. A 1 nm film of organic contaminant belonging to the alkane group will drop the optical transmission at 157 nm by 1%. Further, an acetone residue left on the surface of a calcium fluoride optical element reduces the transmission by 4% at 157 nm. (See, T. M. Bloomstein et al.,

Optical Materials and Coatings at

157 nm, 3676 S.P.I.E. Proceedings 342-9 (1999) incorporated herein by reference). Optical intensity is an important issue as the number of optical elements increases in a lithography tool. It is for this reason that organic contamination can be detrimental to optical elements in 157 nm lithography.

Sources of organic contamination within a lithography tool include out-gassed products from polymer materials and solvents used for degreasing tool parts. Extremely low levels of organic contamination are critical for the exposure path in the lithography tool, and an active purge system and strict material selection are required for those areas of the tool associated with this path. The areas of the lithography tool that are controlled in this manner to keep contamination to a minimum are referred to as clean.

There are areas of a lithography tool for which cleanliness is not critical for operation such as the reticle storage area. Organic contamination is assumed to be present in such areas. However, with use of shorter wavelength lithography tools that have a greater sensitivity to organic contaminants, it is necessary to maintain even the reticle storage area in a clean state. This practice is necessary to avoid introduction of organic contaminates to the exposure system. Thus, an additional area of the lithography tool is added to the areas within which it is critical to maintain a clean environment. This increases the cost of the lithography tool and maintenance thereof Therefore, it is desirable to develop a system to avoid adding the reticle storage area to the critical list of areas of a lithography tool that must be maintained as a “clean” environment.

SUMMARY OF THE INVENTION

The present invention is directed to integrating a cleaning station with a lithography tool to eliminate the need to maintain the reticle storage area (also known as the reticle library) as a clean environment. According to the present invention, the reticle would be cleaned directly prior to use. Therefore, special “clean” storage is not necessary for the reticle library. The reticle cleaning procedure permits a library of reticles inside the lithography tool to be maintained without adding unnecessary environmental constraints. The reticles are retrieved from the reticle library and translated to the cleaning station in which the cleaning process occurs. Upon completion of the cleaning process, the reticle meets the cleanliness requirements for use in the exposure area of the lithography tool.

The reticle cleaning process of the present invention can be conducted at room temperature, atmospheric pressure and in an oxygen-containing environment. Moreover, the reticle cleaning process is performed in situ, thereby permitting the lithography tool to perform exposure using one reticle while the next reticle to be used is being cleaned.

Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and together with the description further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.

FIG. 1

is a block diagram of a lithography tool incorporating a reticle cleaning station.

FIG. 2

is a schematic diagram of a reticle cleaning station.

FIG. 3

illustrates a computer system block diagram for computer controlled monitoring of contaminants to determine optimum cleaning time of reticles in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention is now described with reference to the figures where like reference numbers indicate identical or functionally similar elements. Also in the figures, the left most digit of each reference number corresponds to the figure in which the reference number is first used. While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the relevant art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the invention. It will be apparent to a person skilled in the relevant art that this invention can also be employed in a variety of other applications.

FIG. 1

is a block diagram of a lithography tool incorporating a reticle cleaning station. Lithography tool

100

can be any commercially available lithography exposure tool, such as those tools manufactured by SVG Lithography Systems, Inc. of Wilton, Conn. Particularly a lithography tool operating at any of the following wavelengths:

Wavelengths for Optical Lithography

Optical

Region

Wavelength

I-Line

365 nm

DUV

248 nm

VDUV

193 nm

VUV

157 nm

EUV

 13 nm

(approximately)

Among other features, lithography tool

100

has reticle storage area

102

, reticle cleaning station

106

and exposure area

108

. Multiple reticles are stored in reticle storage area

102

, also known as a reticle library, which according to the present invention need not be maintained as a clean environment. Robot

104

retrieves a reticle from reticle storage area

102

and transports the reticle to reticle cleaning station

106

. Robot

104

can be any commercially available robotic system that can be adapted for use in a lithography tool, such as the RX60 model manufactured by The Staubli Group of Faverges, France. Within reticle cleaning station

106

, the reticle is cleansed of organic contamination as discussed in detail below. Robot

104

then transports the clean reticle directly to the clean environment of exposure area

108

for immediate use. Exposure area

108

may include a light source, a filter, illumination optics, projection optics, an alignment system and a wafer stage as is generally known in the art. Particularly a lithography exposure system as utilized in any one of the Micrascan family of lithography tools manufactured by SVG Lithography Systems, Inc. of Wilton, Conn. that are incorporated herein by reference.

As represented by the schematic diagram of

FIG. 2

, reticle cleaning station

106

is comprised of ultraviolet light source

202

, reticle holding member

204

, oxygen-containing environment

206

and reflectors

208

. Ultraviolet light source

202

includes an assembly of ultraviolet (UV) lamps that have wavelengths between 300 nm and 2 nm. In one embodiment of reticle cleaning station

106

, ultraviolet light source

202

consists of an assembly of UV lamps such as a Hg lamp assembly, which emits light with wavelengths of 184.9 nm and 253.7 nm. In another embodiment of reticle cleaning station

106

, ultraviolet light source

202

consists of an assembly of VUV lamps such as an Xe eximer lamp assembly, which emits light with a wavelength of 172nm. The number of lamps in each assembly is dependent on the area to be illuminated, and is thus application specific as would be apparent to one skilled in the lithography art.

Reticle holding member

204

is located in the reticle cleaning station so as to hold the reticle to be cleaned parallel with and in close proximity to ultraviolet light source

202

. Behind ultraviolet light source

202

are reflectors

208

, which enable uniform forward illumination of the reticle to be cleaned. Reflectors

208

are optional and can be fabricated from a corrosion resistant material such as Alzak, an aluminum material with a corrosion-resistant oxide coating licensed by the Aluminum Company of America of Pittsburgh, Pa.

Reticle cleaning station

106

requires an oxygen-containing environment

206

for the cleaning operation. The reticle cleaning operation consists essentially of UV radiation from light source

202

acting upon diatomic oxygen contained in the oxygen-containing environment

206

and organic contamination from the reticle, thereby creating atomic oxygen and dissociated organic contaminants. At the appropriate UV wavelengths, diatomic oxygen will dissociate into atomic oxygen. The creation of atomic oxygen is a cyclic process that also involves the generation of ozone due to the atomic oxygen combining with diatomic oxygen to form ozone. Ozone will photo-dissociate into atomic oxygen and diatomic oxygen, as represented below.

Atomic oxygen is highly reactive and therefore will also oxidize organic contaminants found on a reticle thereby forming gas phase products, such as carbon dioxide and water. Environment

206

is purged of the gas phase contaminants during the cleaning cycle by purge subsystem

210

. According to an embodiment of the present invention, purge subsystem

210

introduces a pressurized inert gas into reticle cleaning station

106

, which displaces gas phase H

2

O and CO

2

. For example, purge subsystem

210

can comprise a vacuum pump to remove the gas phase contaminants. Appropriate purge systems for semiconductor processing would become apparent to a person skilled in the relevant art given the present description and application.

Thus, as described above and in accordance with the present invention, a reticle cleaning station can be integrated into the lithography tool. A reticle is pulled out of its storage container within the reticle storage area by a robot and translated to the reticle cleaning station. The reticle is positioned to the UV lamp assembly within close proximity of the UV lamps within the reticle cleaning station. The UV lamps are activated for the cleaning process. The duration of this process depends on the chemical makeup of the reticle and extent of contamination of the reticle. The cleaning time can be set a priori or determined automatically by computer controlled monitoring of the level of contaminant being purged. Such computer controlled monitoring can be performed by the computer system described below. Upon completion of the cleaning cycle, the reticle is immediately translated by robot to its final destination, the exposure path within the exposure station of the lithography tool.

The reticle cleaning method described can be utilized with lithography tools of various wavelengths. However, the present invention is particularly useful for cleaning reticles prior to usage in a 157 nm optical lithography system. The reticle cleaning process can be performed at room temperature, atmospheric pressure, and in an oxygen-containing environment and is performed during normal operation of the lithography tool.

Referring to FIG.

3

and as noted above, computer controlled monitoring of contaminants to determine optimum cleaning time can be performed by an example computer system

300

. A computer system

300

includes one or more processors, such as processor

304

. Processor

304

is connected to a communications bus

302

. After reading this description, it will become apparent to a person skilled in the relevant art how to implement the invention using other computer systems and/or computer architectures.

Computer system

300

also includes a main memory

306

, preferably random access memory (RAM), and can also include a secondary memory

308

. Secondary memory

308

can include, for example, a hard disk drive

310

and/or a removable storage drive

312

, representing a floppy disk drive, a magnetic tape drive, an optical disk drive, etc. Removable storage drive

312

reads from and/or writes to a removable storage unit

314

in a well-known manner. Removable storage unit

314

, represents a floppy disk, magnetic tape, optical disk, etc. which is read by and written to by removable storage drive

312

. Removable storage unit

314

includes a computer usable storage medium having stored therein computer software and/or data.

In alternative embodiments, secondary memory

308

can include other similar means for allowing computer programs or other instructions to be loaded into computer system

300

. Such means can include, for example, a removable storage unit

322

and an interface

320

. Examples of such can include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM, or PROM) and associated socket, and other removable storage units

322

and interfaces

320

which allow software and data to be transferred from the removable storage unit

322

to computer system

300

.

Computer system

300

can also include a communication interface

324

. Communication interface

324

allows software and data to be transferred between computer system

300

and external devices. Examples of communication interface

324

include, but are not limited to a modem, a network interface (such as an Ethernet card), a communications port, a PCMCIA slot and card, etc. Software and data transferred via communication interface

324

are in the form of signals which can be electronic, electromagnetic, optical or other signals capable of being received by communication interface

324

. These signals

326

are provided to communications interface via a channel

328

. This channel

328

carries signals

326

and can be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, an RF link and other communications channels. Moreover, computer system

300

can be directly controlled or programmed by a main lithography tool computer (not shown) via communication interface

324

.

Data concerning the contaminant level detected by known chemical detection apparatus can be processed by the computer system

300

via communication interface

324

. The computer system

300

can be programmed to determine the appropriate reticle cleaning time based on this data so as to control robot

104

and reticle cleaning station

106

.

In this document, the terms “computer program medium” and “computer usable medium” are used to generally refer to media such as removable storage device

312

, a hard disk installed in hard disk drive

310

, and signals

326

. These computer program products are means for providing software to computer system

300

.

Computer programs (also called computer control logic) are stored in main memory and/or secondary memory

308

. Computer programs can also be received via communication interface

324

. Such computer programs, when executed, enable the computer system

300

to perform the features of the present invention as discussed herein. In particular, the computer programs, when executed, enable the processor

304

to perform the features of the present invention, such as controlling reticle cleaning time. Accordingly, such computer programs represent controllers of the computer system

300

.

In an embodiment where the invention is implemented using software, the software can be stored in a computer program product and loaded into computer system

300

using removable storage drive

3

12

, hard drive

310

or communication interface

324

. The control logic (software), when executed by the processor

304

, causes the processor

304

to perform the functions of the invention as described herein.

In another embodiment, the present invention is implemented primarily in hardware using, for example, hardware components such as application specific integrated circuits (ASICs). Implementation of the hardware state machine so as to perform the functions described herein will be apparent to persons skilled in the relevant art(s).

In yet another embodiment, the invention is implemented using a combination of both hardware and software.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. All cited patent documents and publications in the above description are incorporated herein by reference.