HK1195135B - Cleanup method for optics in immersion lithography - Google Patents

Description

Cleaning method for immersion lithography optical system

The divisional application is based on the divisional application of the chinese patent application having the application number of 2004800096916, the application date of 2004-4/2, and the title of the invention being "cleaning method for immersion lithography optical system". More specifically, the present divisional application is a divisional application again based on a divisional application having an application number of 201010113560.X, an application date of 2004, 4/2/2004, and an invention name of "cleaning method for immersion lithography optical system".

Reference to related patent applications

This application claims priority to U.S. provisional patent application No.60/462,556, filed on 11/4/2003, and U.S. provisional patent application No.60/482,913, filed on 27/6/2003, the contents of which are incorporated herein by reference.

Technical Field

The present invention relates to an immersion lithography system, and more particularly, to a method for cleaning an optical element that contacts and adsorbs water in an immersion lithography process, in addition to the system.

Background

Immersion lithography systems for providing a liquid into a space between a workpiece, such as a wafer, and a last stage optical element of the optical system for projecting an image of a reticle onto the workpiece are disclosed in WO99/49504, the contents of which are incorporated herein by reference for the purpose of illustrating the general background of the art and some of the general considerations associated therewith. The liquid thus provided can improve the performance of the optical system and the quality of exposure.

The liquid to be provided for light having a wavelength of 193nm may be water, although different liquids may be required for light having other wavelengths. Since the last stage optical element of the optical system is exposed to liquid, it is likely that some liquid will be adsorbed. This possibility is particularly high if the last stage optical element of the optical system is a lens, since calcium fluoride is a common lens material used in lithography systems, whereas it is a hygroscopic material that readily adsorbs moisture from the surrounding environment.

The adsorbed moisture can cause a variety of problems. First, it can alter the refractive properties of the lens or swell the lens and thereby alter the geometry of the lens, thereby degrading the image projected by the lens. Second, it may cause long-term lens deterioration due to chemical effects.

Conventional air immersion exposure lithography systems require that the optical elements be removable for maintenance work, such as cleaning.

However, removing the optical element and resetting it after cleaning or replacing the optical element with a new one is a cumbersome and time consuming operation.

Disclosure of Invention

It is therefore an object of the present invention to provide a system and method that can repeatedly remove water from a lens so that the amount of adsorbed water does not reach a critical value, thereby preventing image degradation and long-term damage to the lens.

It is another object of the invention to provide a system and method that makes it easier to maintain the optical elements of an immersion lithography apparatus, thereby providing a useful lifetime of the optical elements.

The immersion lithography apparatus of the present invention includes a reticle stage for holding a reticle, a stage for holding a workpiece, an optical system including an illumination source and an optical element positioned opposite the workpiece and projecting an image of the reticle onto the workpiece by radiation from the illumination source while defining a gap between the optical element and the workpiece, and a fluid providing device for providing an immersion fluid between the optical element and the workpiece and into contact with each other during an immersion lithography process. The apparatus also includes a cleaning device for cleaning the optical element. In this context, the term "cleaning" is used to mean both removing liquid absorbed into the optical element and removing dust, debris, salts, etc.

Many different types of cleaning devices than those described above may be used within the scope of the invention. For example, it may comprise a cleaning liquid having an affinity for the immersion fluid to be contacted with the optical element. If the immersion liquid is water, ethanol may be used as the cleaning liquid. As another example, the cleaning device may comprise a heat generating device for heating the optical element and/or a vacuum device for creating vacuum conditions on the optical element.

Ultrasonic oscillations can be used to remove adsorbed liquids. An ultrasonic oscillator, such as a piezoelectric transducer, may be attached to the housing of the optical element or placed opposite the optical element so as to emit vibrations through the liquid held in the gap toward the optical element.

Alternatively, cavitation bubbles (cavitation bubbles) may be used to remove adsorbed liquid. A finned gasket is used to generate cavitation bubbles in a liquid held in a gap between the gasket and the optical element.

According to a further embodiment of the invention, the cleaning liquid may optionally be supplied using a nozzle through which the immersion liquid is supplied into the gap between the workpiece and the optical element, by providing a flow-switching device (e.g. a switching valve).

With the same and method of the invention, the cleaning process becomes considerably easier and faster, since there is no longer a need to disassemble the optical element to be cleaned, and the cleaning process increases the lifetime of the optical element.

Drawings

The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic cross-sectional view of an immersion lithography apparatus in which the method and system of the present invention may be employed;

FIG. 2 is a process flow diagram illustrating an exemplary process for fabricating a semiconductor device using the apparatus shown in FIG. 1 in accordance with the present invention;

FIG. 3 is a flowchart of the wafer processing steps shown in FIG. 2 in the fabrication of a semiconductor device according to the present invention;

FIG. 4 is a side view showing a portion of the immersion lithography apparatus of FIG. 1;

FIG. 5 is a schematic side view of a portion of another immersion lithography apparatus having an ultrasonic transducer as a cleaning device;

FIG. 6 is a schematic side view of a portion of another immersion lithography apparatus having a piezoelectric cleaning device under the optical system;

FIG. 7 is a schematic diagram of one example of a piezoelectric device;

FIG. 8 is a schematic side view of a portion of yet another immersion lithography apparatus having two piezoelectric planar elements secured to each other as a cleaning device;

FIG. 9 is a schematic side view of a portion of yet another immersion lithography apparatus having a foam gasket as a cleaning device; and

FIG. 10 is a schematic side view of a portion of another immersion lithography apparatus having a switching device within the fluid supply device.

Throughout the drawings, similar or equivalent components are denoted by the same symbols or numerals in different drawings, and explanation may not be repeated for the purpose of simplifying the explanation.

Detailed Description

FIG. 1 shows an immersion lithography apparatus 100 upon which the method and system of the present invention can be applied.

As shown in fig. 1, the immersion lithography apparatus 100 includes an illumination optical unit 1 including a light source such as an excimer laser unit, an optical integrator (or homogenizer) and a lens, for emitting ultraviolet light IL having a wavelength of 248nm so as to be incident on a pattern on a reticle R. The pattern on the reticle R is projected onto the photoresist-coated wafer W at a prescribed magnification (e.g., 1/4 or 1/5) by a telecentric light projection unit (telecentric projection unit) PL. The pulsed light IL may be selected from ArF excimer laser with a wavelength of 193nm, F with a wavelength of 157nm₂Excimer laser lasers, or mercury lamps i-line with a wavelength of 365 nm. Subsequently, in describing the structure and function of the lithographic apparatus 100, the directions are represented by X-, Y-and Z-axis coordinate systems shown in FIG. 1. For ease of disclosure and explanation, in fig. 1, the light projection unit PL illustrates only the last-stage optical element (e.g., lens) 4 located opposite the wafer W and the cylindrical housing 3 containing the remaining components.

The reticle R is supported on a reticle stage RST that includes a mechanism for moving the reticle R in the X and Y directions and rotating about the Z axis. The two-dimensional position and orientation of the reticle R on the reticle stage RST is detected in real time by a laser interferometer (not shown), and the positioning of the reticle R is influenced by the main control unit 14 based on the detection.

The wafer W is held on the Z stage 9 by a wafer holder (not shown) for controlling the focus position (along the Z axis) and the tilt angle of the wafer W. The Z stage 9 is fixed on an XY stage adapted to move on an XY plane substantially parallel to the imaging surface of the light projection unit PL. An XY stage 10 is set on a base 11. In this way, the Z stage 9 adjusts the focus position (along the Z axis) and the tilt angle of the wafer W by the auto focus and auto alignment (leveling) method for matching the wafer surface with the image surface of the light projection unit PL, and the XY stage 10 is used for adjusting the position of the wafer W in the X direction and the Y direction.

The two-dimensional position and orientation of the Z-stage 9 (and hence also referred to as the wafer W) is monitored in real time by another laser interferometer 13 and with reference to a movable mirror fixed to the Z-stage 9. Control data based on this monitoring is sent from the main control unit 14 to the stage driving unit 15, which is adapted to control the movements of the Z-stage 9 and the XY-stage 10 in accordance with the received control data. In the exposure, the projection light is sequentially moved from one exposure position to another on the wafer W in a step-and-repeat (step-and-repeat) procedure or in a step-and-scan (step-and-scan) procedure according to the pattern on the reticle R.

The lithographic apparatus 100 described with reference to fig. 1 is an immersion lithographic apparatus and is therefore adapted to fill the space ("gap") between the surface of the wafer W and the lower surface of the last stage optical element 4 of the light projection unit PL with a particular kind of liquid (or "immersion liquid") 7, for example water, at least when projecting the pattern on the reticle R onto the wafer W.

The last stage optical element 4 of the light projection unit PL may be detachably fixed to the cylindrical housing 3 and designed such that the liquid 7 contacts only the last stage optical element 4 and does not contact the cylindrical housing 3 because the housing 3 is typically made of a metal material and is easily corroded.

The liquid 7 is supplied to the space above the wafer W under temperature-controlled conditions from the liquid supply unit 5, which may include a tank, a pressure pump, and a temperature regulator (not separately shown), and is collected by the liquid recovery unit 6. The temperature of the liquid 7 is adjusted to be approximately the same as the temperature in the chamber in which the lithographic apparatus 100 is placed. Numeral 21 denotes a supply nozzle through which the liquid 7 is supplied from the supply unit 5. Numeral 23 indicates a recovery nozzle through which the liquid 7 is collected into the recovery unit 6. It should be kept in mind, however, that the structure described above with reference to fig. 1 does not limit the scope of immersion lithography apparatus to which the cleaning method and device of the present invention can be applied. In other words, it is not necessary to mention that the cleaning method and device of the present invention can be applied to many different types of immersion lithography apparatuses. In particular, it should be borne in mind that the number and arrangement of the supply and recovery nozzles 21 and 23 surrounding the light projection unit PL may be designed in different ways in order to establish a smooth flow and a fast recovery of the immersion liquid 7.

The cleaning method for carrying out the present invention, which can remove a part of the liquid 7 such as water, and dust, debris, etc. adsorbed by the last-stage optical element 4 made of a hygroscopic material, is explained below with reference to fig. 1 and 4. After exposing the wafer W with light irradiating the optical unit 1 through the light projection unit PL in the presence of the liquid 7 as shown in fig. 1, the liquid 7 is removed from below the light projection unit PL and the cleaning device 30 and the last stage optical element 4 are brought into contact with each other as shown in fig. 4. In a portable type of embodiment, as shown in fig. 4, the cleaning device 30 may be disposed on the Z-stage 9 or the aforementioned wafer holder, as shown in fig. 4, in place of the wafer W.

Different models and kinds of cleaning devices 30 may be used for the purposes of the present invention. As a first example, the cleaning device 30 may be a container containing a liquid ("cleaning liquid") having a strong affinity with the immersion liquid 7 adsorbed by the optical element 4. If the immersion liquid 7 is water, the cleaning device 30 can contain ethanol because ethanol has a strong affinity with water. Any cleaning liquid may be used as long as it has a strong affinity with the liquid to be removed and does not damage the optical element 4 and its coating. Both surfaces of the optical element 4 are immersed in the cleaning liquid for a period of time up to a level sufficient to remove the adsorbed liquid. The cleaning device 30 is then removed and the optical element 4 is ready for exposure to the liquid 7 again.

As another example, the cleaning device 30 may include a heat generating device and/or a vacuum device (not separately shown). The combination of heat and vacuum on the surface of the optical element 4 causes the adsorbed liquid to change phase to a gas or to evaporate from the surface. The reduction of the density of the liquid on the surface of the optical element 4 allows the liquid 7 absorbed deeper into the element 4 to reach the surface.

Fig. 5 shows a third example in which an ultrasonic transducer (or ultrasonic oscillator) 32 attached to the housing 3 of the light projection unit PL is used. As the ultrasonic transducer 32 (e.g., a piezoelectric transducer) is activated, pressure waves are generated and propagated for cleaning the surface of the optical element 4.

During the cleaning operation of fig. 5, the gap adjacent to the optical element 4 is filled with the immersion liquid 7. In this case, the supply and recovery nozzles can continuously supply and collect the immersion liquid 7, or the supply and recovery nozzles can stop supplying and collecting the immersion liquid 7. Also during the cleaning operation, the optical element 4 can face the surface of the wafer W, the surface of the Z-stage 9 or the surface of another component.

Fig. 6 is a fourth example of the use of an oscillating tool 34 under the optical element to be cleaned. The tool 34 may be shaped similar to the wafer W, with a thickness approximately equal to the thickness of the wafer W, or less than about 0.5-1mm, and may be made entirely of piezoelectric material so that its thickness fluctuates when activated. Because the tool 34 is located below the optical element 4, similar to the wafer W shown in fig. 1, and the gap between the optical element 4 and the tool 34 is filled with the liquid 7, pressure waves are generated in the immersion liquid 7 to clean the optical element.

During the cleaning operation of fig. 6, the gap adjacent to the optical element 4 is filled with the immersion liquid 7. In this case, the supply and recovery nozzles can continuously supply and collect the immersion liquid, or the supply and recovery nozzles can stop supplying and collecting the immersion liquid 7. In other examples, the oscillator 34 may be an ultrasonic transducer attached to the wafer holder on the Z-stage 9 or other component.

Fig. 7 shows another tool 36 having a different configuration with a plurality of piezoelectric transducers 38 supported by a planar support element 39.

Figure 8 shows a further example of a cleaning device having two planar elements 40 of piezoelectric material attached in a face-to-face relationship and adapted to oscillate parallel to each other and 180 deg. out of phase. As a result, the elements 40 are attached to each other to oscillate in the transverse direction, which is shown in an exaggerated manner in fig. 8. The oscillation has nodes at constant intervals at positions where the preventing member 40 is not provided. Element 40 is supported at these nodes on support element 41. When a voltage is applied to these elements 40 to cause oscillation in the above-described manner, an ultrasonic pressure wave is generated and propagated through the liquid 7, thereby cleaning the optical element 4 as desired.

Fig. 9 shows yet another example of a liquid removing system featuring cleaning of the optical element 4 by generating cavitation bubbles. Cavitation bubbles captured and energized by ultrasound are high-temperature, high-pressure microreactors, and the powerful energy released by implosive compression (implosive compression) of the bubbles is thought to be able to pull molecules apart. The example shown in fig. 9 is characterized by comprising a pad 43 having wings projecting upward and rapidly moving horizontally as indicated by the arrows below the optical elements 4 while filling the gap 9 therebetween with the foaming liquid 7 (means for moving the pad 43, not shown). With the pad 43 so moved, the wings serve to agitate the liquid 17 and create cavitation bubbles for cleaning the optical elements.

Fig. 10 shows a different approach to the problem of cleaning the last stage optical element 4 by applying a cleaning liquid on its bottom surface using a nozzle 21 of the same origin as the immersion liquid 7 is supplied. For this purpose, a switching valve 25 is interposed between the supply nozzle 21 and the liquid unit 5, thereby enabling the immersion liquid 7 and the cleaning liquid to selectively pass through the supply nozzle 21.

It is to be remembered again that the cleaning method and system according to the invention can be applied to immersion lithography apparatuses of different types and kinds, i.e. with a different number of source nozzles. The above-described switching valve need not be provided for each source nozzle, but may be provided for a group of source nozzles.

When the cleaning liquid is thus supplied through the supply nozzle 21, it may be appropriate to place the wafer W itself under the optical element 4 or the pad 18 can provide an appropriate gap therebetween. This embodiment of the invention is advantageous in that the cleaning process can be performed using the same nozzles already used for providing the immersion liquid.

Although the various methods are described separately above, it is not necessary to describe them at this time but it should be understood that they may be used in combination, although not shown separately in the drawings. For example, the pad 43 having wings shown in fig. 9 may be substituted for the pad 18 shown in fig. 10. In other words, the above examples do not limit the scope of the present invention, and many modifications and variations are possible within the scope of the present invention. For example, a polishing pad similar to that used in chemical mechanical polishing processes can be used for this purpose. The cleaning procedure shown in fig. 4-10 may be performed with ultraviolet light. The optical element 4 may be irradiated with light. The light may be normal exposure light from the illumination optics 4, or other light having a wavelength suitable for cleaning. In other examples, ultraviolet light for cleaning may be used instead of the cleaning procedure shown in fig. 4 to 10, and may be used under the condition that the gap adjacent to the optical element 4 is filled with the immersion liquid 7 from the liquid supply unit 5. All such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention.

Again, it should be noted that any of the above cleaning methods may be used to remove both the immersion liquid absorbed by the last stage optical element and the salts, deposits, dust and debris that may accumulate. The term cleaning is therefore used herein to include both of these phenomena.

The process of manufacturing a semiconductor device using an immersion lithography apparatus including a liquid ejecting and recovering system embodying the present invention is described below with reference to fig. 2. In step 301, the functional and performance characteristics of the device are designed. Next, in step 302, a patterned mask (reticle) is designed according to the previous design steps, and in a parallel step 303, the wafer is fabricated from a silicon material. At step 304, the mask pattern designed at step 302 is exposed onto the wafer fabricated at step 303 by a lithography system, such as the system described above. At step 305, the semiconductor device is assembled (including the dicing process, bonding process and packaging process) and then the final device is inspected at step 306.

Fig. 3 illustrates a detailed flow example of the above step 304 in the manufacture of a semiconductor device. In step 311 (oxidation step), the wafer surface is oxidized. In step 312(CVD step), an insulating film is formed on the wafer surface. In step 313 (electrode forming step), an electrode is formed on the wafer by vapor deposition. At step 314 (ion implantation step), ions are implanted within the wafer. The aforementioned steps 311-314 form a pre-processing step during wafer processing and may be selected as desired at each step.

At each stage of wafer processing, when the above-described pretreatment step is completed, the following post-processing step is performed. During post-processing, photoresist is applied to the wafer at step 315 (photoresist forming step). Next, in step 316 (exposure step), the circuit pattern on the mask (reticle) is transferred onto the wafer using the exposure device described above. Then, in step 317 (developing step), the exposed wafer is developed, and in step 318 (etching step), a portion (exposed material surface) other than the residual photoresist is removed by etching. In step 319 (photoresist removal step), unnecessary photoresist remaining after etching is removed. A plurality of circuit diagrams can be formed by repeating these processing and post-processing steps.

While the lithography system of the present invention has been described in terms of several preferred embodiments, it is contemplated that alternatives, permutations and various substitute equivalents are also within the scope of the present invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following claims be interpreted as including all such alterations, permutations, and various substitute equivalents as fall within the true spirit and scope of the present invention.

Claims (7)

1. A cleaning method for an immersion lithography apparatus in which a workpiece for lithography is positioned on a stage during an immersion lithography process, a projection optical system having an optical element is above and opposite to the workpiece, a gap is provided between the optical element and the workpiece, the method comprising the steps of:

placing a cleaning device on the table during a cleaning process; and

supplying a liquid between the optical element and the cleaning device on the stage by a nozzle during the cleaning process, the optical element being in contact with the supplied liquid during the cleaning process,

wherein the nozzle supplies an immersion liquid to a gap between the optical element and the workpiece during the immersion lithography process,

wherein the supply of the liquid and the collection of the supplied liquid continue during the cleaning process.

2. The cleaning method according to claim 1, wherein said cleaning process is performed after said immersion lithography process.

3. The cleaning method according to claim 1, wherein the liquid supplied during the cleaning process is water.

4. An immersion lithography apparatus wherein a gap is provided between an optical element and a workpiece for lithography during an immersion lithography process, the immersion lithography apparatus comprising:

a stage on which the workpiece for lithography is held during the immersion lithography process and on which a cleaning device is placed during a cleaning process;

a projection optical system having the optical element, which is above and opposite the workpiece during the immersion lithography process; and

a nozzle arranged to supply an immersion liquid to the gap between the optical element and the workpiece on the stage during the immersion lithography process and arranged to supply a liquid to the gap between the optical element and the cleaning device on the stage during the cleaning process, the optical assembly being in contact with the supplied liquid during the cleaning process,

wherein the supply of the liquid and the collection of the supplied liquid continue during the cleaning process.

5. The apparatus of claim 4, wherein said cleaning process is performed after said immersion lithography process.

6. The apparatus of claim 4, wherein the liquid supplied during the washing process is water.

7. A device manufacturing method, comprising:

exposing the wafer by the apparatus of any one of claims 4-6; and

the exposed wafer is developed.