Enhancing alignment in lithographic apparatus device manufacture

In a lithographic apparatus, a scanning mechanism is coarse compared with precise patterns to be exposed onto a substrate. In order to ensure that the image and the substrate are aligned at some point in time, an oscillation is imparted to either the substrate table, or to a device that aligns the image, such as a mask table. The oscillation frequency is chosen to compliment a maximum alignment error. The frequency of a radiation pulse is arranged to coincide with the image and the substrate being most accurately aligned. The radiation pulse of the image may be timed to coincide with the alignment without the use of the imparted oscillation.

BACKGROUND

1. Field of Invention

The present invention relates to a lithographic apparatus and a method for manufacturing a device using lithography. Specifically, the present invention relates to the alignment of a substrate or wafer in the path of a patterned radiation beam.

2. Description of Related Art

The scanning of the pattern may follow what is known as a “meander”, which is a possible path taken by the scanning stage to ensure that the entire target portion is included in the scan and is therefore exposed.

In order for the correct portion of the substrate to be exposed, the mask and the substrate must always be aligned. In other words, the accuracy of the product position on the substrate as described above is at least in part determined by the relative position of the substrate and the patterning device during exposure of the pattern. In particular, the accuracy (or lack thereof) of a position of an image of the pattern on the substrate at the moment of exposure is dependent on the relative positional error of the substrate support and the patterning device support at the very instant a radiation beam is transmitted through the patterning device onto the substrate surface. Position errors impact overlay (i.e. the accuracy of one exposed layer on another) and critical dimension uniformity (i.e. the width of product structures on the substrate surface).

However, there are always alignment errors because the movements of the mask (table) and the substrate (support) necessarily move more slowly than the exposure radiation. The supports (or tables) that support the substrate and the patterning device respectively are controlled such that they move with minimum positional error and with constant velocity. In the meantime, the radiation (which is often in the form of a laser) fires pulses at a constant frequency (pulse repetition rate). Accidental or unintentional movement in any of the mechanical devices may occur just at the timing of the radiation and although the radiation beam (or laser) may be in the correct position, the mask and substrate may not, at that precise moment, be perfectly aligned.

There is an increasing requirement for smaller and more precise patterns to be exposed on the substrate. As products become smaller, relative movements of the substrate table and the patterning device table become less tolerable and a cause of errors. In other words, it becomes increasingly difficult to keep the support positional errors below decreasing tolerances. One reason for this is that the supports (or tables) are mechanical components that do not allow instantaneous repositioning or very rapid movements. In addition, disturbance forces like acoustics or noise in electronic components limit the obtainable accuracy. However, the support that supports the patterning device and the support that supports the substrate both need to be aligned during the radiation pulse (i.e. during the exposure of the pattern on the substrate surface) and it is in this alignment that accuracy errors may occur. These errors are often caused by unintentional motion of one of the supports in response to acoustical and/or mechanical vibrations by electronic sources.

SUMMARY

This section is for the purpose of summarizing some aspects of the present invention and to briefly introduce some preferred embodiments. Simplifications or omissions may be made to avoid obscuring the purpose of the section. Such simplifications or omissions are not intended to limit the scope of the present invention. Consistent with the principles of the present invention as embodied and broadly described herein, the present invention includes

It is desirable to ensure accurate alignment between a radiation pulse, a patterning device (on its support), and a substrate (on its respective support). It is alternatively desirable to ensure accurate alignment between a projected image and the substrate.

According to an aspect of the invention, there is provided a lithographic apparatus comprising: an illumination system configured to condition a radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, wherein the lithographic apparatus further comprises an oscillation device configured to impart a controlled oscillation perpendicular to an optical axis of the projection system to at least one of the patterning device support and the substrate table in the projection system.

This controlled oscillation is imparted such that, in use, a probability of the patterned radiation beam being aligned with the target portion of the substrate at some time during the oscillatory movement is increased.

According to an aspect of the invention, there is provided a lithographic apparatus comprising: an illumination system configured to condition a radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, wherein: the lithographic apparatus further comprises a relative position detector configured to detect a relative position between the patterned radiation beam and the target portion of the substrate; and the illumination system is further configured to impart a timed pulse to the radiation beam, the pulse being timed to coincide with a minimal positional offset between the patterned radiation beam and the target portion of the substrate.

According to an aspect of the invention, there is provided a method of projecting a patterned radiation beam onto a substrate comprising imparting a controlled relative oscillatory motion between the projected patterned radiation beam and the substrate, the relative oscillatory motion being in a direction perpendicular to a direction of projection of the projected patterned radiation beam.

According to an aspect of the invention, there is provided a device manufacturing method comprising projecting a patterned beam of radiation onto a substrate, and oscillating an image of the patterned radiation beam with respect to a substrate on which the patterned radiation beam is projected in a direction perpendicular to a projection direction of the patterned beam of radiation. There may further be provided a device manufactured according to the method above.

DETAILED DESCRIPTION

The invention will be better understood from the following descriptions of various “embodiments” of the invention. Thus, specific “embodiments” are views of the invention, but each does not itself represent the whole invention. In many cases individual elements from one particular embodiment may be substituted for different elements in another embodiment carrying out a similar or corresponding function. The present invention relates to

Embodiments of the present invention may be applied to any of the modes described above. An embodiment relates to the timing of a laser pulse forming the radiation beam being controlled such that the laser pulse is fired when the image embodied in the patterning device and the target portion of the substrate are aligned. An embodiment of the present invention relates to controlling the relative positions of the patterning device and the substrate as well as the timing of pulses of the radiation beam such that an image of the pattern that is exposed on the substrate is positioned as accurately as possible. Such control may be performed by a feedback control loop or a feed-forward control loop. Either the substrate table WT or the support structure MT of the patterning device (e.g. a mask table), or both, may be moving (e.g. scanning along a meander, or stepping between static positions) in order to expose a pattern over the full surface of the substrate. Errors may be introduced during this movement (or may be present in the static position). Specifically, the movement actuated by the long- or short-stroke actuators that move the support structure MT or the substrate table WT may not be accurate enough for very small or precise patterns to be exposed accurately.

Typically, the mask table MT and the substrate table WT are complex mechanical structures with large masses that are moved small distances and that exhibit internal resonances. The positions of the tables MT, WT are controlled by measuring (e.g. using an interferometer) the actual position of each table (taking into account multiple degrees of freedom) and comparing the measured positions with pre-set motion set points. The mask table MT and substrate table WT positions are measured generally at a rate that is independent of the pulse rate of the radiation beam. For example, the table MT, WT positions may be measured at a rate of 5 kHz, whereas the radiation beam pulse may have a frequency of 5 kHz. When the positions are found to contain errors, a correcting actuator force is produced to minimize the error by moving at least one of the tables. However, using an actuator (which needs to be able to quickly accelerate large masses) to move the large masses very small distances in order to minimize a positional error is difficult below a certain threshold minimum error. Furthermore, there are other causes of movement in the tables MT, WT. These movements may be vibrations caused by the high forces used to accelerate the tables, or acoustic vibrations (air pressure variations induced by the scanning motion), as well as sensor and amplifier noise. The present invention seeks to overcome these difficulties.

The radiation that is used to expose a substrate is often a laser and so reference to the radiation pulses herein below may be to laser pulses or laser firing. Typically, a laser pulse frequency is 4 kHz or more, and a substrate support scanning speed is of the order of 500 mm/s. At a typical slit width of, for example, 5 mm, each portion of a pattern is exposed for 0.01 s, and so includes on the order of 40 laser pulses.

In an embodiment of the invention, variation in the timing of laser pulses is used as an extra variable to improve the position of the image (of the pattern) on the substrate during exposure. Only when the substrate is aligned with the mask (within permissible tolerances) is the laser triggered. This is practical if it is certain that a position “close to” the original laser pulse timing (without adjustment for alignment errors) is available at which the relative table position error (i.e. the misalignment between the mask table and the substrate table) is zero. The laser pulse timing may be brought forward or delayed if the mask table and the substrate table are aligned at a time other than the expected moment. However, in an embodiment the laser pulse timing may only be brought forward or delayed by a specific margin so as not to decrease throughput (i.e. the overall time taken to expose the target portion of the substrate).

In order to take advantage of the variability of the laser pulse timing, an embodiment of the present invention further involves the oscillation of one or both tables to increase the probability that the mask table and the substrate table will be perfectly aligned at some point in time and space during the oscillation. At this point in the oscillation, the laser pulse is fired. This has the advantage of increasing the throughput because the system will not require the laser pulse to be delayed or brought forward by as large a margin as if there were no oscillation.

By the induction of an oscillation, what is meant is that a controlled oscillation is applied to either the patterned radiation beam or the substrate so that a relative oscillation between the two is created. The oscillation may be induced by introducing a controlled cyclical movement or a perturbation to a table or to the radiation beam. The perturbation may be to an existing movement. For example, the oscillation may be induced in addition to the meander of the substrate (with respect to the radiation beam). The meander may be carried out by moving the substrate table that supports the substrate in a meander path while the radiation beam is held in one position. The oscillation may then be superimposed onto the meander movement of the substrate table, or the oscillation may be applied to the radiation beam. The oscillation is more than a simple vibration, as a vibration is often uncontrolled—i.e. an amplitude may be initially set and the vibration is then allowed to find its natural resonance. The imparted oscillation of the present embodiment on the other hand is carefully controlled so that it has a predetermined and constant frequency that is linked to a maximum frequency error of the device or table being oscillated and also to a frequency of the laser pulses.

In other words, in order to ensure that the relative table error (i.e. the error in the position of the substrate table relative to the projected image) is as close to zero as possible, a sinusoidal oscillation is applied to the projected image (or the substrate with respect to the image) at the nominal laser repetition rate, for example, 4 kHz. The timing of this extra sinusoidal oscillation or motion is such that at every nominal laser pulse instant, the added sinusoidal motion has a zero crossing. By zero crossing, it is meant that a sinusoidal graph such as shown inFIG. 3or4crosses a zero line representing the middle of the oscillation between the maxima and minima of the sinusoid. At this zero crossing, the velocity of the moving table or device is at its maximum, and in the absence of positional disturbances, the tables MT, WT are (preferably perfectly) aligned (as the zero crossing is midway between the changes of direction at the maxima and minima of the sinusoid). If there are positional disturbances, the alignment of the tables MT, WT will occur at a point on the sinusoid outside of the zero crossing (or the zero crossing will effectively be moved, seeFIGS. 3 and 4) and the timing of the laser pulse will coincide with the alignment (i.e. the new zero-crossing).

The amplitude of the added oscillation or sinusoidal motion is of the order of two (2) to ten (10) times the amplitude of the maximum relative table error. A maximum relative table error may, for instance, be in the region of 10 nm. In this case, therefore, the added oscillation may have a frequency of 4 kHz and an amplitude of 20 to 100 nm. The oscillation is controlled so that it is considerably shorter in duration than the exposure time of the target portion of the substrate. For example, one exposure takes approximately 60 ms, while the oscillation period imparted to the system is of the order of 0.25 ms or less. More importantly, the oscillation period is substantially equal to a nominal laser pulse firing frequency.

The oscillation may be induced by injecting an extra signal to a mask table or substrate table, as will be described below with reference toFIG. 2. The oscillation may be imparted to the radiation beam using an oscillating plane plate. In a lithographic apparatus using a reflective or catadioptric projection system, the oscillation may be imparted to the radiation beam by oscillating a reflector.

FIG. 2shows certain portions of the lithographic apparatus ofFIG. 1. Specifically, the source SO is shown to send a laser pulse through the illuminator IL and then towards a mask MA on a mask table MT. The radiation pulse passes through the mask and is thereby patterned. This patterned radiation beam then passes through the projection system PS and finally onto the substrate W surface. The substrate W is held by a substrate table or substrate support WT.

FIG. 2further shows two switches, V and DR. The first switch V acts as an oscillation device in this embodiment. The oscillation device may not be a switch, but simply an oscillation device having a single output. This oscillation device may impart an oscillation or sinusoidal motion to either the mask table MT via output VM or to the substrate table WT via output VW, as indicated by the dashed line inFIG. 2.

Oscillations may be applied to one or both of these portions of the apparatus, or to another portion of the apparatus that will cause oscillation of the image relative to the substrate W. Of course, there may be no switch at all and simply a single output oscillator attached to any one (or more) of the devices.

The second switch ofFIG. 2functions as a detector DR. Detector DR detects the motion or the position of either the mask table MT via input DM or the motion or the position of the substrate table WT via input DW. Of course, the detector may be a single motion detector or position detector associated with a single device, rather than being a switch.

The detector feeds back information about the position of the mask table MT and/or the substrate table WT to the source SO. The source then causes a laser pulse to be fired just at the moment when the image (i.e. the patterned radiation beam), which has traveled through the mask M and the projection system PS, is as close to exactly in line with the desired position on the substrate W as possible. In this way, the pattern may more accurately be exposed onto the substrate W.

The effect of adding the extra sinusoidal motion or oscillation is that somewhere close to the nominal (i.e. default) laser timing (at which time the laser would have fired had there been no added oscillation), there is hopefully always a time instant where the relative position of the substrate with respect to the mask is exactly zero. In fact, even the oscillation of the tables or optical device may not cause an exact alignment, but it is induced in such a direction that the probability of alignment is increased relative to the case with no added oscillation. The actual laser pulse is then fired at this time instant, within acceptable tolerances. The timing for the firing of the laser pulse is shown inFIG. 3.

InFIG. 3, the actual substrate table position data may be input via a detector DR and may have, for example, a maximum position error of 8 nm. To the substrate support error, a 100 nm, 4 kHz oscillation, A, is added.FIG. 3shows a sinusoid with its maxima and minima at 100 nm, this sinusoid being the added oscillation and is labeled “A”. If the substrate support position error is zero, the laser will fire at each zero crossing OX of the clean 100 nm sinusoid, corresponding to an exact laser pulse timing of 250 μsec. In the case shown inFIG. 3, however, the laser pulse timing is adjusted based on the positional error of the substrate support. Sinusoid B shows the sum of the added oscillation and a substrate table error that starts at 8 nm and decreases over a period of about 250 μs. The difference between the two sinusoids A and B gives rise to the difference required in the laser pulse timing. In the example ofFIG. 3, the laser needs to fire 2.7 μsec later than the nominal to ensure a zero position error on the substrate.

A smaller added oscillation amplitude is possible for the same substrate table error, but this would result in a requirement of a larger deviation of laser timing from the nominal value. For example, if the added oscillation has a 20 nm amplitude as shown by sinusoid A2inFIG. 4, a 13 μsec laser timing adjustment is needed.

FIG. 4shows the actual substrate support motion plus the added oscillation as sinusoid B2. The zero crossing OX2is shown in the middle of the sinusoidal wave. The timing of the laser pulse is shown across the axis and it is shown that a larger deviation is from the nominal value (i.e. a larger gap along the x-axis between the sinusoids A2and B2) is required inFIG. 4than inFIG. 3. This is because the actual table position error is a larger proportion of the added oscillation in the example ofFIG. 4than in the example ofFIG. 3.

As mentioned above with respect toFIG. 2, there are several ways of adding the oscillation to the various portions of the lithographic apparatus. A method of adding, for example, a 4 kHz oscillation is to add 4 kHz to the movement of one of the tables itself (i.e. using existing actuators). However, this would have a relatively large impact on the movement of the table and would require a large force to move the large mass of one of the tables MT, WT.

Now follows the method of calculating the laser pulse timing. A separate oscillator may alternatively be used to add an independent oscillation to either or both of the tables.

The substrate table positional error changes with relatively low frequency. Therefore, measuring the position with, for example, a 10 kHz frequency is more than enough to predict the time instant at which the relative error between the tables MT, WT is zero. In normal operation, there are position controllers that minimize the positional error of the tables and these are controlled by a control computer. With a fixed clock rate, the motion control algorithm is calculated. A typical motion controller clock rate is 5 or 10 kHz. The laser pulses are fired at a different clock rate which is determined by the capabilities of the exposure laser that is used. A typical laser pulse clock rate is 4 kHz. This difference in clock rates between the motion controllers and the laser pulses results in the laser pulse typically not coinciding with a “clock tick” of the motion controller. At a specific motion controller clock tick, it is known how much time will pass until the next laser pulse would nominally be fired. Also, at this clock tick, the position measurement data of the tables and of the oscillatory device are known. Because the rate of change of the substrate support error is much slower than the clock frequencies, the measured position deviation at a clock tick may be assumed to remain constant until the next laser firing pulse. Therefore, a laser pulse timing correction for the forthcoming laser pulse can be calculated by the motion controllers on the basis of the measured positions, and their deviations, from the desired positions. This timing correction is then sent to the laser pulse timing control electronics.

In other words, at the last motion controller clock tick before the nominal laser pulse time instant, the required pulse timing variation is calculated.

As mentioned above, inFIG. 3, sinusoid B shows the added oscillation plus a substrate table error with maximum value 8 nm. The difference between the two graphs A and B gives rise to the difference required in the laser pulse timing. The above method may be applied to positional errors in the x-axis, the y-axis, or even in both. The method is best suited for errors in only one of the x and y axes because it is unlikely that these are both zero at the same moment. However, to deal with both x and y errors, oscillation in a combined x/y direction is possible. Instead of considering only the x axis or only the y axis, the time instant may be selected that minimizes the vectorial error √{square root over (x2+y2)}.

The advantages of the invention are now discussed. The image positional error on the substrate at the moment of the laser pulse is decreased, leading to smaller mean average (MA) and mean standard deviation (MSD) values (i.e. smaller high frequency and low frequency errors) and hence better overlay and critical dimension measurements. This method allows the use of less accurate table positioners, for example using only a long-stroke actuator in the scanning of the substrate table WT. This greatly reduces table complexity and cost. Finally, the use of the oscillation in combination with the adjusted laser pulse timing allows the throughput of the substrate(s) not to be compromised for the sake of accuracy.

CONCLUSION

Various embodiments of the present invention have been described above. It should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made from those specifically described without departing from the spirit and scope of the invention. Thus, the breadth and scope of 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.