NL1040348C2 - Drying device for use in a lithography system. - Google Patents

Description

Drying device for use in a lithography system

Background of the invention 1. Field of the Invention

The present invention relates to a drying apparatus for drying a target such as a wafer, for use in a lithography system for projecting an image pattern on to a target surface such as a wafer. 2. Description of the Related Art

In known lithography systems, the target, such as a wafer, to be patterned is subjected to incidence of photons or charged particles such as ions and electrons. So as to realize high precision patterning of the target, it is to be firmly bonded or connected to a target table (or wafer table) by means of which the target is moved relative to the source of incidence, at least if position measurement of said target is performed via said target table. It is preferably also to remain in position relative to said table during handling of the target, e.g. during insertion and removal of the target to and from the position where it is to be processed.

Various solutions exist for meeting the above requirements, e.g. by means of electromechanical clamping or clamping by capillary action using a fluid and causing the wafer undesirably to become wet. An example of such a clamping system utilising capillary clamping is for example known from International Application WO2009/011574.

In the known system a target is included in a lithography system by means of a target table and clamping means are present for clamping said target on said table. Said clamping means comprises a layer of stationary liquid, included at such thickness between target and target table that, provided the material of the liquid and of the respective contacting faces of the target and target table, a pressure drop arises. The stationary fluid may be for example water. After patterning, the target and target table are removed from the lithography system and unclamped, i.e. the target and target table are separated.

In the known clamping system, after unclamping the target needs to be dried before returning back to further processing, such as a coater/developer track. The target backside may be wet with liquid droplets (such as water droplets) after unclamping from the capillary clamping table. Liquid spots left on the wafer surface can cause oxidation that damages components on the wafer.

Besides the known clamping system, other stages of the lithography process such as coating, etching and cleaning may leave liquid on the target when the wafer is processed. In a lithography system, especially in a vacuum system, these liquid droplets inside system should be avoided as these may contaminate the wafer during further processing or interfere with the rest of the system. The wafer thus should be dried before it is further processed.

Since the target is to be processed in a lithography system, there are several requirements placed on the drying apparatus. The target should be dry enough, i.e. no liquid or droplets remain, since any remaining liquid or dropiets may still negatively influence the lithography process.

Further, the drying apparatus should not add contamination to the target whilst drying, again to not negatively influence the lithography process. Since a target such as a wafer passes through the entire lithography system, any contamination present on the target will spread throughout the entire system.

After drying the target may need to be processed further. For some stages of processing, it is required that the temperature of the target is controlled, thus the drying apparatus should not raise or lower the temperature of the target in a manner that disrupts the lithography process

Due to limited space, which is also highly expensive, available in cleanrooms where lithography processes typically take place, the drying apparatus should preferably be of a small and compact size relative to the lithography system.

Several ways of drying are known in the art. A first known way of drying is spin drying, where the target or wafer is spun and the centrifugal force removes the liquid or droplets. Spin drying typically takes place at speeds of 2000 rpm. Ideally, wafers are spun off-axis e.g. spun around an axis that does not coincide with the center of the target. Disadvantages of this method are that spinning the wafer, especially off-axis, introduces unwanted vibrations. Also, off-axis spinning increases the footprint of the drying apparatus, which is not wanted. If on-axis drying in used, e.g. the wafer is spun around the center of the wafer, it was found that large residual droplets are left on the wafer.

Another know way of drying wafers is by using an "air knife". In such systems, the wafer passes through an high speed gas flow (e.g. nitrogen, N2) to remove droplets by blowing them away from the surface of the target. Typical flow speeds for the gas are 50 m/s to 200 m/s. In this manner droplets are blown away, not evaporated. This type of drying is often employed in laboratories, and is not typically used in industrial lithography applications. A disadvantage is the use of an air knife place high aerodynamic forces are exerted on wafers. Typical wafers are of 300mm size and can fail when exposed to the forces exerted by the air knife. Further, since the liquid is blown away and not evaporated, the liquid may contaminate other parts of the system.

Another known way of drying wafers which is commonly used in lithography processing is Marangoni drying. In this process, a wafer is vertically retracted from a liquid bath, typically water. IPA (isopropyl alcohol) vapour and heated N2 is made to flow over the wafer, causing a surface tension gradient in the liquid. This allows the liquid film to smoothly recede from the wafer since the presence of a gradient in surface tension will naturally cause the liquid to flow away from regions of low surface tension. The heated N2 flows across the wafer to evaporate the IPA. A typical Marangoni drying speed is 6 mm/s leading to a total drying time of 50 s for a 300 mm wafer. Disadvantages of Marangoni drying include the complexity and size of the drying system since the wafers have to be vertically pulled and the relatively low drying speed.

It has been suggested to combine Marangoni drying with an air knife, where the wafer is passed through an IPA saturated air knife. It is however not ciear if the surface tension gradient may also occur in the turbulent aerodynamic conditions in the air knife. Also, residual condensed IPA is blown away by the air knife in an uncontrolled manned, and can cause contamination of the system.

Marangoni drying may further be combined with a form of spin drying (then called Rotagoni drying) where the IPA displaces the liquid on the wafer under centrifugal force, the IPA then evaporates. Besides the disadvantages outlined above under spin drying and Marangoni drying, this combined technique is normally only useful for the top wafer surface.

The use of wipers has also been suggested for drying wafers, it was found however that liquid drops may remain on the wafer using a wiper. The removed liquid remains on the wiper and may fall off, contaminating the system. Also, bottom of the wafer is touched by the wiper, which is undesirable and may lead to contamination of the wafer.

Applying heat is a known way of drying, but is relatively slow for wafer drying and may leave spots on the wafer. Conversely, freeze drying may be used, however the very low temperatures may have adverse effects on the wafer.

Brief Summary of the invention A goal of the present invention is to overcome the problems as identified in the known systems, in particular to provide a system to dry a wafer after unclamping.

This goal is achieved by providing a drying apparatus comprising a drying device which comprises a first slit and a second slit arranged in close proximity of the target, a gap being present between the target and the dryer, the first slit included for supplying pressurised gas into said gap directed to the target, the second slit included for discharging said liquid by means of said gas away from the target. A dryer transverses along the backside of the wafer and has two slits along its width. A first slit being an exit slit, blowing dry air or N2 against the wafer backside. A second slit, through which the air or N2 flow is sucked by vacuum back into the dryer.

As the drying device traverses the length of wafer, droplets are removed by the suction of the second slit, i.e. the air suction slit, at the leading edge of the dryer. The first slit i.e. the air exit slit provides an air or N2 flow parallel to the wafer in the dryer movement direction. It ensures that droplets are pushed towards the air suction slit, leaving a dry wafer backside behind.

By ensuring that the suction slit flowrate is equal to or greater than the exit slit flow rate, droplets are not blown away into the outside environment. Instead the suction ensures that all droplets are removed in a controlled manner.

This dryer can alternatively also work upside-down, useful for drying the wafer topside or wafer table surface.

Brief description of the drawings

These and other objects of the invention will be further elucidated with reference to the figures, in which:

Figure 1 in sectional view schematically illustrates a known clamping system comprising liquid, capillary included between two plate like structures;

Figure 2 by sectional view of part of a lithography system, schematically represents a practical implementation of a wafer clamp;

Figure 3a shows a top view of a layout of lithography system according to an embodiment of the invention;

Figure 3b schematically shows a cross-sectional side view of a portion of the lithography system of Figure 3a;

Figure 4 schematically represents a practical implementation of the idea underlying the present invention;

Figure 5 schematically represents bottom view of a practical implementation of the idea underlying the present invention .

Figure 6 schematically shows an alternative arrangement of the dryer .

Description of illustrative embodiments

Figure 1 in sectional view schematically illustrates a known clamping system for use in a known lithography system, comprising a lithography target, here in the form of a wafer 1, which normally moves relative to e.g. a charged particle beam column of a lithography apparatus, or other kind of beam source for lithography, by means of an actuated target table or chuck, not indicated in this drawing. Between a top side of such target table 2 and said target 1, a volume of liquid 3 in included capillary. To this end the target 1 and top surface 2 have a mutual nominal distance of gap height h. The volume of liquid, according to the invention preferably water, is such that the radius of the target, as taken in top view, is virtually met by a radius R of the capillary included liquid 3. In any case, the radius of an incircle of the target is at least met by the radius of an excircle fitting the volume of liquid within the boundary of the target. The thus included liquid 3 forms a liquid surface 4, alternatively denoted fluid interface 4 at it's outer periphery which, as taken in the section according to figure 1, is generally concavely shaped, due to adhesive connection of the liquid to the target 1 and the top side 2 of a target table respectively. This concave surface 4 tends to maintain its shape at pulling the target and target table apart, and depends on pressure differential.

Figure 2 illustrates a practical elaboration on the known principle and schematically shows part of a lithographic system, including a target 1 embodied by a wafer, a top side 2 of a wafer table 8 or chuck part, a liquid 3 usually in the form of water. Optionally, a so called "Viton" or rubber 0-ring 9 is present. The optional 0-rings 9 seal off liquid vapour evaporating from the gap containing liquid 3, which is especiaiiy an issue in the vacuum environment in which such lithography means may be applied, by being inserted in a rim part of reduced height of the wafer table 8. The top side of the O-ring is set to a level corresponding in height to that of, and preferably being slightly higher than that of burls 7 on the top side of wafer table 8.

Figure 3a shows a top view of a layout of lithography system 300 comprising a group of lithography system units according to an embodiment of the invention. Hereinafter, the layout may be referred to as lithography system 300 or cluster 300.

Such lithography systems are described for example in U.S. Patent Nos. 6,897,458 and 6,958,804 and 7,019,908 and 7,084,414 and 7,129,502, U.S. patent application publication no. 2007/0064213, and co-pending U.S. patent applications Serial Nos. 61/031,573 and 61/031,594 and 61/045,243 and 61/055,839 and 61/058,596 and 61/101,682, which are all assigned to the owner of the present invention and are all hereby incorporated by reference in their entirety.

Figure 3b schematically shows a cross-sectional side view of a portion of the lithography system 300. In this particular embodiment, the lithography system 300 comprises a group of ten lithography system units. The lithography system units are arranged back-to-back in two rows of five. Directly adjacent to the cluster 300, floor space is reserved as service area 305. Each lithography system unit comprises a lithography apparatus 301 that is contained in its own vacuum chamber, with one side of each vacuum chamber facing a lithography system unit in the other row, while the opposing side faces the surroundings of the cluster 300, in particular the service area 305.

In case of a charged particle lithography apparatus, the vacuum chamber preferably comprises all elements that enable lithography processing, including a charged particle source, a projector system for projecting charged particle beamlets onto a substrate to be patterned, and a moveable substrate stage .

The side of the lithography system unit facing the surroundings comprises a load lock system 310 for transferring substrates into and out of the vacuum chamber, and also comprises an access door 330 that can be opened for service purposes .

The lithography system units are provided with a door 330 at the same side as the load lock system 310. The door 330 may be removably attachable, and may be removable in its entirety, for example by using a transfer unit 340. The transfer unit 340 may be arranged to support the door 330 and may comprise one or more transfer elements 345, such as wheels or rails. The lithography apparatus 301 may be supported by a supporting structure 335 for positioning the lithography apparatus at an elevated position.

The lithography system 300 thus comprises a plurality of lithography system units having a load lock system 310 and a door 330 facing the surroundings, more in particular facing the service area 305 surrounding the lithography system 300. Due to the "outward" orientation of the load lock systems 310 and doors 330, the lithography system units, including the lithography apparatuses 301 within the vacuum chambers, are directly accessible from the service area 305. Direct access simplifies servicing of the lithography system 300, and may reduce the downtime of the lithography system or parts thereof. Opening a single specific vacuum chamber for servicing can be done without affecting the throughput of other lithographic system units within the lithography system 300 .

The load lock system 310 may be integrated into the door 330. Integration of the load lock system 310 and the door 330 reduces the amount of material used in manufacturing the lithography system unit. A portion of the door 330 may be directly used as one of the side walls of the load lock system 310. The material reduction has the advantage that the door and load lock system combination is easier to handle during servicing. Furthermore, as less material is needed during manufacturing, the costs of manufacturing the lithography system are reduced as well.

The lithography system 300 further comprises a substrate supply system 315. The substrate supply system 315 is arranged to receive substrates to be processed by the lithography system 300, and to provide these substrates to the lithography system units for processing. This can effectively mean that the substrate supply system 315 provides the substrates to a preparation system 320 for preprocessing purposes. After patterning, the substrate supply system 315 may collect the patterned substrates. The use of a substrate supply system 315 enables the lithography system 300 to efficiently cooperate with other equipment in the fab as it allows for a relatively easy replacement of presently used lithography systems.

Before entry into the vacuum chamber, a substrate typically undergoes the actions of clamping, pre-alignment and pump down. In this context, ciamping is defined as providing a substrate on a substrate support structure to form a single structure, hereafter referred to as "clamp". Furthermore, the term "clamped substrate" is used to refer to a substrate being clamped to a substrate support structure. Pre-alignment relates to aligning the substrate and/or clamp such that patterning can be performed onto a predetermined portion of the substrate in a certain orientation. Pump down relates to the step of reducing the pressure surrounding the substrate to minimize contamination and to reduce the influence of the substrate on the vacuum chamber pressure upon insertion into the lithography apparatus 301.

After the patterning action performed by the lithography apparatus 301, the substrate is typically exposed to a venting action, and an unclamping action, i.e. separating the substrate from the substrate support structure. In between the venting and unclamping actions, the substrate may be transferred.

The load lock system 310 forms an interface to a vacuum environment within the vacuum chamber. The system 310 is typically used for the pump down action and the venting action described above. For this purpose, the load lock system 310 comprises one or more chambers in which the pressure can be regulated. The load lock system 310 may comprise a single chamber suitable for both pump down and venting actions. Alternatively the system 310 comprises separate chambers for pump down and venting. For the pump down action the system 310 comprises pumps for pumping down the pressure within a chamber to a reduced pressure, e.g. a vacuum suitable for transfer of the clamped substrate and substrate support to the lithographic apparatus 301. For the venting action the ioad iock system 310 comprises vents for venting a chamber to increase the pressure after processing of the clamped substrate in the lithographic apparatus 301.

Clamping and/or unclamping may be performed in the preparation systems 320. Alternatively, the clamping may be performed at a different location prior to providing the substrate to the preparation systems 320, for example within the common supply system 315. In yet another alternative, clamping and/or unclamping may be performed within the load lock system 310.

Clamping and unclamping may be performed in separate units, but may also be executed in the same unit. Hereinafter the expression "clamping unit" refers to a unit for clamping and/or unclamping.

After unclamping the wafer may need to be dried if the clamping systems according to figures 1 and 2 was used. In systems where the top of the wafer has a liquid layer to increase the numerical aperture of the optical system, the wafer may also need to be dried after processing in the chamber. Other lithography processes such as cleaning of wafers or wet etching of wafers may also require the wafer to be dried. For these processes the drying unit according to the invention is also suitable. The clamping unit in the preparation system therefore comprises a drying apparatus according to the present invention. It is advantageous to include the dryer in the clamping unit, as this is the first place the wafer is after processing in the lithography system, reducing the risk of cross contamination.

Figure 4 represents a practical implementation of the idea underlying the present invention.

The drying apparatus 402 traverses along the wafer 401, for example in direction 406. The wafer may also be traversed along the dryer. A gas such as dry air or N2 is blown onto wafer 401 through a first slit 404 at a pressure Pin. As the dryer 402 and wafer 401 move relatively to each other the gas is blown into the gap 407 in between the dryer and the wafer. As a lower pressure Pout exists in the second slit 405, the removed water droplets are carried by the gasflow (indicated by the three arrows in Fig.4) into second slit 405 to transfer the droplets out of the gap 407 between the dryer and the wafer. In this manner liquid droplets 403 are blown away and removed from the wafer.

It is essential that no flow exists in other directions along the surface of the wafer, for example along the wafer outside of the flow of gas, since the droplets carried by the flow can contaminate the lithography system. In order to achieve this the outlet pressure Pout should be low enough, whilst the gap 407 is relatively small. When gap 407 is to large, the optimal flow cannot easily be achieved. Conversely, the gap 407 should not be so small that the wafer is sucked onto the dryer and becomes stuck, possibly damaging the wafer. It was found that a minimum gap height of approximately 0.5 mm is optimal, with an airflow of about 25 m/s. The width of slit 404 was found to be optimal at 0.5 mm to supply the required flow, while slit 405 has an optimal width of 1 mm to allow the water droplets to exit. This setup in practice results in that larger liquid droplets, ie. droplets larger than approximately 1 mm in diameter are blown away, whilst smaller droplets evaporate within a short time of about 30 seconds after drying.

It is advantageous that the wafer is not physically contacted by the drying apparatus, reducing the risk of contamination of the wafer.

Figure 5 schematically represents bottom view of a practical implementation of the idea underlying the present invention.

The wafer bottom 501 (shown from the bottom) is moved relative to the dryer 502 along the length 505 of wafer 501. The gas flows out of the first slit 503 into the gap (not shown) between the dryer and the wafer, removing liquid droplets (not shown). The gas flow exits the gap through the second slit 504. In this example the slits extend along the entire width of wafer 501, however it was found that it is not always necessary to dry the entire wafer. In certain processes, for example after the wafer was clamped according to the process of Figures 1 and 2, only a small area 506 may remain wet. In these circumstances it the dryer may applied using smaller slits only covering the width of the wet spot 506. It is then also not necessary to traverse the entire length 505 of the wafer, again merely drying the length of wet spot 506. This has the advantage that a smaller drying apparatus can be used.

It was found that when using the drying apparatus, in certain circumstances the top of the wafer may also become wet at the edge of the wafer where remaining droplets are blown from the bottom of the wafer to the top when the dryer traverses the edge of the wafer.

Figure 6 schematically shows an alternative arrangement of the dryer for preventing the droplets migrating to the top of the wafer. Wafer 601 is traversed in direction 605 by dryer 602 drying the wafer in the manner of figures 5 and 6. A second arrangement 603 is present comprising a third slit 606 adapted for supplying gas on top of the wafer at the edge where droplets may migrate. When the pressure provided by the gas flow is sufficient, droplets from the bottom are prevented from migrating and are carried away from the wafer by the second slit as in the previous examples. Typically, the gasflow supplied by the third slit is supplied from about 15 mm above the wafer. This proved to be sufficient.

As in figure 5, in certain cases it may only be required to to supply gasflow to part of the top edge of the wafer.

Although some embodiments of the invention have been described with reference to a lithography system comprising ten lithography system units, the number of lithography system units within a lithography system may vary. For example, instead of ten lithography system units, any other number of lithography system units above one may be used.

The invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention, which is defined in the accompanying claims .

Claims (15)

A drying device for use in a lithography system for drying at least a portion of the surface of a first side of a planar target object such as a wafer (401), comprising a drying device for removing liquid or drops (403) thereof from the first side of the target target object, wherein the device (402) comprises a first gap (Pin) and a second gap (Pout) which are arranged in close proximity to the target object and liquid therein, such that a gap (407) is present is between the target object and the drying apparatus, wherein the first gap is included for supplying pressurized gas into the gap, and wherein the second gap is included for discharging the gap under a lower pressure (Pout) from that gap (407) liquid through the gas away from the target object, the first and the second slit being configured to allow a substantially parallel in the gap current directed at the target surface from the first slit to the second slit of pressurized gas along said first side of the planar object, and wherein the drying device is adapted for a relative movement between the target object and the device, which relative movement and which flow of gas have a corresponding direction, or at least a directional component, in particular such that drying, or at least removal of adhering liquid, is made possible, more particularly that the relative movement and liquid removal take place along the underside of the target object. Device as claimed in claim 1, further comprising means for moving the drying apparatus and the target object relative to each other. Device as claimed in claim 2, wherein the first slit and the second slit are arranged substantially parallel to each other. Device as claimed in claim 1, 2 or 3, wherein the first and second slit are adapted to supply the gas and to discharge the gas and liquid in a direction perpendicular to the target object. The device of claim 4, wherein between the first and second slit, the gas flows substantially parallel along the surface of the target object. The device of claim 1, further comprising a third drying gap adapted to blow a gas onto the target object for drying a portion of the edge of the target object. The device of claim 6, wherein the further slit is included to dry the top of the target object. Device as claimed in any of the foregoing claims, wherein the drying device is adapted to dry only a part of the edge of the target object. The device of claim 8, wherein the drying apparatus is adapted to dry an area on the edge of the target object with a maximum width of 45 mm. Device according to any one of the preceding claims, wherein the gas is air or nitrogen (N 2). Device as claimed in any of the foregoing claims, wherein the distance between the drying device and the target object is approximately 0.5 mm. Device as claimed in any of the foregoing claims, wherein the flow velocity in the gap is approximately 25 m / s. Device as claimed in any of the foregoing claims, wherein a third gap is included for supplying gas to the top of the wafer. A charged particle lithography system for transferring a pattern to a target object comprising a drying device according to any one of the preceding claims, the system further comprising: a beam generator for generating a plurality of beams; - a modulating device for deflecting the plurality of bundles in accordance with pattern data; - a bundle stop element for selectively blocking the deflected bundles to form patterned bundles; a projection system for projecting the patterned beams on the target surface; and - a positioning system for positioning the target object and the projection system relative to each other, in particular - a clamping station for clamping the target object with the aid of liquid on a holder, more particularly comprising a clamping station in which the drying device is included. A method for drying a target object such as a wafer by removing liquid or drops thereof from the target object, comprising the steps of: - providing a first slit and a second slit; - providing a planar target object such as a wafer; - providing a gap between the first and second slit and the wafer; supplying gas under pressure into the gap from the first gap in the direction of the target object; and - draining the liquid or drops thereof away from the target object into the second slit by means of the gas.