Ultrasonic distance sensors

A lithographic apparatus comprises an illumination system, a support constructed to support a patterning device, and a projection system. In pixel grid imaging, a large number of small optical spots are imaged onto the substrate surface using a micro-lens array (MLA). The z position of the MLA is adjustable in order to focus the spots on the substrate surface and/or to compensate for differences in height of the substrate surface. The focusing adjustment is based on an output of an ultrasonic distance sensor provided in the vicinity of the substrate surface.

BACKGROUND

1. Field of the Invention

The present invention relates to ultrasonic distance sensors and is concerned more particularly, but not exclusively, with the use of such ultrasonic distance sensors in lithographic apparatus.

2. Related Art

In maskless lithography it is known to project the patterned beam onto a target portion of the substrate using a projection system that comprises an array of lenses arranged such that each lens receives and focuses a respective portion of the patterned beam. Each lens of the array of lenses thus projects a respective spot of radiation onto the substrate, and the array of lenses collectively projects a radiation pattern on the substrate. In one type of lithography machine, imaging of the resist layer on the substrate is affected using pixel grid imaging. To achieve this, a large number of small optical spots are imaged onto the resist layer using a series of micro-lens arrays (MLA). The z position of each MLA or group of MLA's is adjustable in order to focus the spots on the surface of the substrate. The focusing of the MLA's is adjustable, so as to compensate for differences in height of the substrate surface. Such focusing adjustment of the MLA's can be effected on the basis of the output of level sensors provided in the vicinity of the substrate surface in a feed forward mode. The effectiveness of conventional systems is limited by an accuracy of such distance sensors.

Therefore, what is needed is a system and method using a more effective distance sensor.

SUMMARY

According to one embodiment of the present invention, there is provided an ultrasonic distance sensor for monitoring the height of a substrate surface. The sensor comprises an ultrasonic emitter system, an ultrasonic receiver system, and a signal processor. The ultrasonic emitter system emits a first transmitted beam towards the substrate surface and a second transmitted beam. The ultrasonic receiver system receives a first reflected beam produced by reflection of the first transmitted beam from the substrate surface and a second reflected beam produced by reflection of the second transmitted beam. The signal processor processes signals received from the ultrasonic emitter system and ultrasonic receiver system indicative of time differences between the emission and receipt of the transmitted and reflected beams in order to monitor the height of the substrate surface.

According to one embodiment of the present invention, there is provided a lithographic apparatus comprising an illumination system, a support, a projection system, and a ultrasonic distance sensor. The illumination system conditions a radiation beam. The support is constructed to support a patterning device. The patterning device patterns the beam. The projection system projects the patterned beam onto a target portion of the substrate. The ultrasonic distance sensor monitors the height of a surface of the substrate, and comprises an ultrasonic emitter system, an ultrasonic receiver system, and a signal processor. The ultrasonic emitter system emits a first transmitted beam towards the substrate surface and a second transmitted beam. The ultrasonic receiver system receives a first reflected beam produced by reflection of the first transmitted beam from the substrate surface and a second reflected beam produced by reflection of the second transmitted beam. The signal processor processes signals received from the ultrasonic emitter system and the ultrasonic receiver system indicative of the time differences between the emission and receipt of the transmitted and reflected beams in order to monitor the height of the substrate surface.

According one embodiment of the present invention there is provided a method of monitoring the height of a substrate surface comprising the following steps. Emitting a first ultrasonic transmitted beam towards the substrate surface. Emitting a second ultrasonic transmitted beam. Receiving a first reflected beam produced by reflection of the first transmitted beam from the substrate surface. Receiving a second reflected beam produced by reflection of the second transmitted beam. Processing signals indicative of the time differences between the emission and receipt of the transmitted and reflected beams in order to monitor the height of the substrate surface.

According to one embodiment of the present invention, there is provided a device manufacturing method comprising the following steps. Transferring a pattern from a patterning device onto a surface of a substrate, while adjusting the focusing of the image of the pattern in dependence on variation in the height of the substrate surface. The height of the substrate surface is monitored by the following method. Emitting a first ultrasonic transmitted beam towards the substrate surface. Emitting a second ultrasonic transmitted beam. Receiving a first reflected beam produced by reflection of the first transmitted beam from the substrate surface. Receiving a second reflected beam produced by reflection of the second transmitted beam. Processing signals indicative of the time differences between the emission and receipt of the transmitted and reflected beams in order to monitor the height of the substrate surface.

The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers may indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number may identify the drawing in which the reference number first appears.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1schematically depicts a lithographic apparatus, according to one embodiment of the present invention. The apparatus comprises an illumination system (illuminator) IL, a support structure (e.g., a mask table) MT, a substrate table (e.g., a wafer table) WT, and a projection system (e.g., a refractive projection lens system) PS. The illumination system IL conditions a radiation beam B (e.g., UV radiation or white light). The support structure MT is constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM that accurately positions the patterning device in accordance with certain parameters. The substrate table WT holds a substrate (e.g., a resist-coated wafer) W and is connected to a second positioner PW that accurately positions the substrate in accordance with certain parameters. The projection system PS projects a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

The support structure supports, i.e., bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, for example, whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure can be a frame or a table, for example, which can be fixed or movable as required. The support structure can ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein can be considered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section, such as to create a pattern in a target portion of the substrate. This can include any static or dynamic patterning device, as would become apparent to one of ordinary skill in the art upon reading this description.

In one example, the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate. For example, if the pattern includes phase-shifting features or assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

In various examples, the patterning device can be transmissive or reflective. Examples of patterning devices include, but are not limited to, masks, programmable mirror arrays, grating light valves, programmable LCD panels, and the like. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

In one example, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus can be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

Referring toFIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus can be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source can be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, can be referred to as a radiation system.

The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W.

With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted inFIG. 1) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan.

In general, movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW.

In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short-stroke actuator only, or can be fixed.

In one example, mask MA and substrate W can be aligned using mask alignment marks M1, M2and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they can be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks can be located between the dies.

Pixel Grid Imaging Mode: the pattern formed on a substrate is realized by subsequent exposure of spots formed by a spot generator that are directed onto a patterning array. The exposed spots have substantially a same shape. On substrate the spots are printed in substantially a grid. In one example, the spot size is larger than a pitch of a printed pixel grid, but much smaller than the exposure spot grid. By varying intensity of the spots printed, a pattern is realized. In between the exposure flashes the intensity distribution over the spots is varied.

Combinations and/or variations on the above described modes of use or entirely different modes of use can also be employed.

In one example, the lithographic machine is used in the manufacture of flat panel displays. In this example, the lithographic apparatus projects an image of a mask onto a photosensitive resist layer on the upper surface of a substrate. In one example, the image is exposed using pixel grid imaging using a series of MLA's, the z position of which can be adjusted in order to focus the optical spots on the resist layer.

In this example, The image plane should coincide with the resist layer on the substrate (or, in other words, the resist should be in focus) in order to transfer the smallest details (about λ/NA, where λ is the wavelength of the illuminating radiation and NA is the numerical aperture of the lens system) on the mask that are reproducible with the optical design. If the resist layer is out of focus, the image will be blurred.

As regards the range over which the image is sharp, the depth of focus (about λ/NA^2) is small in a typical UV lithography machine (e.g., a wavelength λ=350 nm, numerical aperture NA=0.2, and depth of focus about 9 μm). As the flatness of the stage, and the thickness of the substrate will vary more than the typical depth of focus, the position of the substrate or the image plane have to be adjusted during an exposure to compensate for variation in the height of the substrate surface (that is the upper surface of the resist layer on the substrate). In order for such adjustment to be effected it is necessary to measure the position of the image plane and the height map of the substrate. The level sensor (LS) measures the height map of the substrate on the stage.

Height maps can be measured with distance sensors based on various principles. In one example, the sensor is based on measuring the time-of-flight of an ultrasound pulse that is generated by a transducer, travels to the resist layer on the substrate, is reflected at the surface of the resist layer, and then travels back to the transducer where it is detected.

If the distance between the transducer and the upper surface of the resist layer is h, the time-of-flight t will be given by:
t=2*h/vsound where vsound is the sound velocity

Thus, if t is measured, and vsound is known, it is possible to calculate h.

In theory the ultrasound pulse will not only reflect on the resist surface, but also at the upper and lower surfaces of the substrate itself. The reflection coefficient of a sound wave when going from a first medium having an acoustic impedance RA1to a second medium having an acoustic impedance RA2is:
(RA1−RA2)^2/(RA1+RA2)^2

For example, see section 6.7 in “The Measurement, Instrumentation and Sensors Handbook”, John G. Webster, CRC Press 1999 ISBN 0-8493-8347-1, which is incorporated by reference herein in its entirety.

The acoustic impedance RA is given by ρ*vsound, where ρ is the specific density of the medium through which the acoustic wave travels and vsound is the sound velocity in the medium (air, liquid or solid). For air the acoustic impedance RA about 300, whereas, for liquids and solids, the acoustic impedance RA is about 1.5*10^6, so that the sound wave is almost completely reflected by the air-to-resist transition, so that only the distance between the transducer and the resist surface is measured.

The distance h between the transducer and the resist surface can be calculated from the time-of-flight t using the expression:
h=vsound/tif the sound velocity in air vsound is known.

The sound velocity depends on the air density that in turn depends on the relative humidity, the pressure and the temperature of the air and the frequency of the ultrasound.

Although vsound can be calculated if the air density is known, a simpler solution is for a reference measurement to be performed by the ultrasonic sensor in addition to the measurement in relation to the resist surface. In this case, the reference measurement uses the same transducer frequency as the other measurement, and the reference measurement is done close to the other measurement so that the air through which the two measurements are made has the same properties.

Exemplary Ultrasonic Distance Sensors

FIG. 2shows an ultrasonic distance sensor, according to one embodiment of the present invention. The system includes a first transducer1for emitting a first transmitted beam5towards the substrate surface3(the upper surface of the resist layer) and a second transducer2, immediately adjacent to the first transducer1, for emitting a second transmitted beam7towards a reference surface4. The first transmitted beam5is reflected from the substrate surface3, such that the beam6reflected from the surface3is transmitted back towards the first transducer1. The second transmitted beam7is reflected from the reference surface4, such that the beam8reflected from the surface4is transmitted back towards the second transducer2. The first transducer1detects receipt of the beam6, and the second transducer2detects receipt of the beam8. A signal processor (not shown) is provided for processing signals received from the first transducer1and the second transducer2indicative of the times-of-flight t1and t2between the emission and receipt of the transmitted and reflected beams5,6and7,8in order to monitor the height of the substrate surface3in the manner described below.

The first transducer1measures the time-of-flight t1between the transducer1and the substrate surface3and the second transducer2measures the time-of-flight t2between the transducer2and the reference surface4, both measurements being made through air with the same composition. If the distance href between the second transducer2and the reference surface4is known, it therefore follows that vsound can be calculated from the expression:
vsound=(href/2)/t2

so that the distance h to be measured between the first transducer1and the substrate surface3can be calculated by the signal processor from the expression:
h=t1*vsound/2=href*t1/t2 which is independent of vsound.

FIG. 3shows an ultrasonic distance sensor, according to one embodiment of the present invention. In this embodiment, the second transducer2and the reference surface4are located such that the second transmitted beam7transmitted by the second transducer2towards the surface4and the beam8reflected from the surface4are transverse to the first transmitted beam5transmitted by the first transducer1towards the substrate surface3and the beam6reflected from the surface3. The measurements are again made through air of substantially the same composition, and the determination of the height h of the substrate surface by the signal processor is undertaken in the same manner as described above

FIG. 4shows an ultrasonic distance sensor, according to one embodiment of the present invention. In this embodiment, the first transducer1and the second transducer2are mounted with a known height difference d (the distance of the second transducer2from the substrate surface3not being known). The signal processor determines the distance h between the first transducer1and the substrate surface3from the signals received from the first transducer1and the second transducer2indicative of the times-of-flight t1and t2between the emission and receipt of the transmitted and reflected beams5,6and7,8in a rather different manner, the reflected beams6and8both being reflected from the substrate surface3in this case.

In this embodiment, the time-of-flight t1between the transducer1and the surface3is given by t1=2*h/vsound, and the time-of-flight t2between the transducer2and the surface3is given by t2=2*(h+d)/vsound, from which vsound =2d/(t2−t1), so that the distance h to be measured between the first transducer1and the substrate surface3can be calculated by the signal processor from the expression:
h=d*t1/(t2−t1) which is independent of vsound.

In this embodiment, the height difference d is known. For example, d may be known by mounting the transducers1and2at a measured distance apart. Alternatively, the height difference d can be determined by a calibration step in which the substrate is measured at two different heights h and h+dh with the difference dh between these heights being known. In this calibration step the signal processor calculates the height measurements d and d+dh between the first transducer1and the substrate surface3for these two different heights from the measured times-of-flight t1and t2with the substrate at height h and the measured times-of-flight t1′ and t2′ with the substrate at height h+dh using the expressions:
h=d*t1/(t2−t1)
h+dh=d*t1′/(t2′−t1′)

from which d can be calculated.

FIG. 5shows an ultrasonic distance sensor, according to one embodiment of the present invention. In this embodiment, only a single transducer1′ is used for measurement of the times-of-flight t1and t2between the emission and receipt of the transmitted and reflected beams5,6and7,8. The ultrasonic beam travelling from the transducer1′ to the substrate surface3should have a certain diameter, so as to make it possible for parts of the beam to be reflected by the reference surface4and by the substrate surface3. When the signal processor processes the reflected sound wave, the travel time ts of the part of the beam reflected by the substrate surface3and the travel time tr of the part of the beam reflected by the reference surface4can be distinguished.

from which can be derived the expression to be used by the signal processor:
h=href*(ts/tr) which is independent of vsound.

FIG. 6shows an ultrasonic distance sensor10, according to one embodiment of the present invention. In this embodiment, a single transducer is used for carrying out the sensing technique as just described with reference toFIG. 5. In this case, the ultrasonic beam from the transducer11is partially reflected from a reference reflector12positioned at a known distance from the transducer11, and partly passes the reflector12to be reflected from the resist layer at the surface14of the substrate. The transducer11receives two echoes, one from the reference reflector12and one from the substrate surface14, and the travel times tr and ts at which these echoes are received at the transducer can be used as shown above to determine the height of the resist surface whilst compensating for temperature and humidity variations of the air between the transducer and the substrate.

In one example, utilizing only one transducer can allow for a cost savings as a result of the fact that only one transducer is required. However, in order to have room for the reference reflector, the transducer is spaced from the substrate surface at a greater distance (a few mm greater) than the arrangements in which two transducers are used.

In one example, using two transducers allows for errors due to process dependent behavior of the sensing arrangement to be cancelled out as they will be the same for both transducers. However, both transducers need to measure at the same part of the substrate so that the transducers have to be placed one behind the other in the scanning direction.

In one example, for high accuracy a high frequency ultrasonic transducer is used with a good coupling efficiency to air. The transducer has a wide bandwidth to achieve a good pulse response, allowing accurate travel time measurement as well as minor ringing to allow a short measuring range and high pulse repetition rates. Generally commercially available ultrasonic distance measurement systems use narrowband transducers that are not suitable for the required application. For maximum resolution, the optimum center frequency of the required transducer should be as high as possible, at least a few hundred kHz. However, at high frequencies, air absorption increases dramatically, for example from about 160 dB/m at 1 MHz to over about 2000 dB/m at 4 MHz. This corresponds to a greater than about 20 dB return loss at about 10 mm, corresponding to the minimum free working range of about 5 mm. The maximum transducer center frequency is thus limited to about 4 MHz.

The transducer aperture (active diameter) affects the beam shape, as well as the transducer footprint on the substrate, that is the area over which the distance is averaged. In one example, a pencil beam with negligible beam spreading is used, if this allows sufficient spatial resolution.

FIG. 7is a block diagram of the electronic circuitry associated with a distance sensing arrangement, according to one embodiment of the present invention. A sensing arrangement comprises dual sensor units20and21, each unit containing two individual ultrasonic transducers and associated front-end electronics. In one example, each sensor unit contains two piezoelectric transducers each with a pre-amplifier/limiter unit. The two transducers are offset in the vertical direction by a calibrated amount, so as to allow the determination of the sound speed by the difference in travel time between the two transducers. The sensor units20and21are connected by a cable to a central electronics unit containing a shared ultrasonic pulser22, which supplies triggering pulses to the sensor units, and post amplifier/filter units23,24,25and26for the different transducers.

The outputs of the post amplifier/filter units23,24,25and26are supplied to the channels of a four-channel high frequency transient recorder27(e.g., an analog-to digital (A/D) converter and memory). A microprocessor28controls the measuring process and communicates with the remainder of the lithographic machine in dependence on the measurement results received by the recorder27. For each sensor head, the sensing arrangement runs two individual, independent processes, namely the measurement process carrying out data acquisition and analysis, and the communication process which handles the interface with the main machine, including input of control parameters and output of measurement data.

FIG. 8shows a measuring process for a single sensor head, according to one embodiment of the present invention. A data acquisition/measuring process starts with a start routine30followed by, for each of the two channels A and B corresponding to the two transducers, a transmit routine31in which the pulser generates an electronic transmit pulse forcing the corresponding transducer to emit a short ultrasonic pulse. At the same time, an acquire routine32is triggered to start the transient recorder. After the echo signals received by the transducers have been recorded by the transient recorder (e.g., simultaneously for the two channels), the processor software transfers a selected time window of the results for each channel to memory in a transfer routine33. This determines in a trigger routine34the echo arrival time from the data in the time window. The travel time for each channel is the amount of time elapsed between the start of the transmit pulse and the arrival of the echo (either the first echo or a specific multiple).

Another software algorithm monitors the placement of the window in relation to the travel time correcting it where necessary for the next transmission/reception sequence to provide a track routine35for tracking gradual variations in distance and/or sound speed. In a mode routine36, statistics of the travel time measurements are monitored to set the mode parameter accordingly, namely one of: (i) the initialisation mode entered upon startup in which echoes are looked for in a wide time window and the statistics of the calculated travel time are monitored, (ii) the normal mode which is entered after stable operation has been achieved and in which outliers exceeding about +/−3 σ limits are discarded, and/or (iii) a locking mode which is entered after a selected number of outliers have been detected close together and in which outlier removal is disabled, thus enabling rapid response to steps in measuring distance. Upon completion of the locking process, the system reverts to normal mode.

The measured travel time is entered in a circular buffer37(e.g., overwriting data recorded previously), and is also used to recompute the average over the last n samples in the buffer by adding the new value while removing the oldest value. From the travel times for channels A and B from the buffers37for the two channels, the sound speed is computed and stored in a third circular buffer38. This implementation ensures the availability at any time of the latest measured value, with outliers removed and averaged over the last n measurements, with minimal computational overhead, and with an averaging filter length n anywhere between 1 and the buffer length.

When triggering on the first echo, a value for n of about 20 should be adequate to achieve the required accuracy. As the statistics between channels can differ slightly, depending on the range offset, for example, n can be set separately for each channel and for the sound speed in the third buffer. This facility is especially important for this last case, since the range offset is only a fraction of the individual measuring distances, so that for this buffer it is estimated that n should probably be set to approximately the square of the ratio between the range offset and the individual ranges.

Separately from the data acquisition/measuring process, a distance output process is provided by which, when a request for data is received from a digital interface39for the rest of the machine, the most recent values are retrieved by a readout routine40from the averages of channels A and B and the sound speed and used in a calculate routine41to calculate the ranges for A and B and supply them to the digital interface.

In addition to the ultrasonic transmission and detector electronics, a thermocouple sensor is provided for detecting the air temperature so as to allow for temperature compensation.

Although specific reference can be made in this text to the use of lithographic apparatus in the manufacture of integrated circuits (ICs), it should be understood that the lithographic apparatus described herein can have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as synonymous with the more general terms “substrate” or “target portion,” respectively.

In one example, the substrate referred to herein can be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein can be applied to such and other substrate processing tools. Further, the substrate can be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein can also refer to a substrate that already contains multiple processed layers.

The term “lens”, where the context allows, can refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

CONCLUSION