WO2013063104A1

WO2013063104A1 - Air gauges comprising dual-range differential pressure sensor

Info

Publication number: WO2013063104A1
Application number: PCT/US2012/061675
Authority: WO
Inventors: Michael Sogard; Sandy Lee
Original assignee: Nikon Corporation
Priority date: 2011-10-25
Filing date: 2012-10-24
Publication date: 2013-05-02

Abstract

Position-sensing devices include first and second gas passageways connectable to a source of pressurized gas. A gas-releasing probe is connected to the first passageway. The probe is positionable relative to a surface and configured to release a first flow of the gas, flowing through the first passageway, toward the surface while a second flow of the gas flows through the second passageway. A differential-pressure (DP) sensing device is connected between the first and second passageways. The DP-sensing device has first and second DP sensors connected in parallel. The first DP sensor produces a gas-pressure measurement within a first dynamic -pressure range, and the second DP sensor produces a gas-pressure measurement in a second dynamic-pressure range. The resulting position measurement has greater dynamic range and higher resolution than would otherwise be produced with either the first DP sensor or the second DP sensor alone.

Description

AIR GAUGES COMPRISING DUAL-RANGE DIFFERENTIAL PRESSURE

SENSOR

Cross-Reference to Related Applications

This application claims priority to, and the benefit of, U.S. Provisional

Application No. 61/628,214, filed on October 25, 2011, which is incorporated herein by reference in its entirety.

Field

This disclosure pertains to, inter alia, air gauges useful for measuring height or, generally, position of an object in a precision system, such as for measuring wafer or substrate height in a microlithography system, without contacting the object.

Background

Various precision systems used in manufacturing and/or measurement require accurate positioning of a workpiece or other object relative to a tool or other process implement. In some systems, proximity sensors are used for this purpose. One group of proximity sensors, providing tantalizing prospects of extreme accuracy (in the nanometer range), are the so-called "air gauges." Air gauges are used in, for example, certain microlithography systems for measuring wafer height during calibrations of autofocus systems, while eliminating measurement errors that may otherwise be caused by layer patterns on the wafer surface and by thin-film effects. Air gauges are particularly useful in this context because they are non-intrusive, do not contact the object, and are extremely accurate.

There are several basic configurations of air gauges. One basic configuration detects changes in air flow accompanying changes in object height. Another basic configuration detects changes in internal pressure (relative to a reference pressure) in an air-gauge circuit accompanying changes in object height.

An example of a conventional air gauge that detects changes in air flow is discussed in U.S. Patent Publication No. 2005/0268698, in which the air gauge comprises a central conduit including a mass flow controller connected to an upstream air supply. Downstream of the mass flow controller the central conduit splits, effectively at a "Y" junction, into a measurement conduit and a reference conduit. The measurement conduit and reference conduit each include a respective flow restrictor and respective "probe" (nozzle or other appropriate opening) through which air is discharged onto a respective surface in close proximity to the probe. Coupled to and extending between the measurement conduit and the reference conduit is a bridge circuit that comprises a mass-flow sensor that measures air- flow in either direction through the bridge conduit. The mass-flow sensor has two ports, one serving as a "reference" port and the other serving as a "measurement port." The reference port is pneumatically connected to the reference conduit, and the measurement port is pneumatically connected to the measurement conduit. Thus, air flow from the central conduit splits at the "Y" junction so that a portion of the flow is through the measurement conduit, and the remaining portion of the flow is through the reference conduit. Air passing through the reference and maintenance conduits exits the respective probes. The reference probe is positioned relative to a reference surface, and the measurement probe is positioned relative to a

measurement surface such as a wafer surface. Each probe is separated from its respective surface by a respective gap, called the "reference gap" and the

"measurement gap," respectively. The reference gap is usually known while the measurement gap represents the height or distance to be measured. If the gaps are exactly equal, there is usually no net flow of air through the bridge conduit.

Unequal gaps cause corresponding flow (volumetric rate and direction) of air through the mass-flow sensor, which detects and quantifies the air flow in either direction through the bridge conduit. A corresponding signal produced by the mass- flow sensor can be used in a control circuit.

An example of an air gauge that detects changes in air pressure is discussed in U.S. Patent No. 5,540,082. In the Ό82 patent document, air in the reference branch is discharged as a controlled air bleed (not requiring a reference surface) instead of through a reference probe. Hence, no reference probe is required. The configuration of this circuit 10 is schematically depicted in FIG. 1. The depicted system 10 comprises a regulated air supply 12 providing a flow of air that is split between a bleed conduit 14 and a measurement conduit 16. The bleed conduit 14 includes a respective orifice restrictor 18, and the measurement conduit 16 includes a respective orifice restrictor 20. Downstream of each orifice restrictor 18, 20 is a respective connection 22, 24 to a bridge conduit 26 including a differential pressure (DP) sensor 28. The DP sensor 28 measures the pressure difference between the connection 24 in the measurement conduit 16 and the connection 22 in the reference conduit 14, and produces a corresponding analog electrical signal. The electrical signal is normally passed through an amplifier and converted to a corresponding digital pressure signal usable by the system of which the air gauge is a part.

The bleed conduit 14 in FIG. 1 also includes a second orifice restrictor 30, which is adjustable for setting an appropriate constant bleed rate of air from the bleed conduit. The measurement conduit 16 continues to the probe 32, from which air is discharged toward a surface 34 of an object 33. The surface 34 is separated from the probe 32 by a gap 36, which is the distance to be measured. With this air gauge, the pressure difference is directly affected by the pressure at the connection 24 in the measurement conduit and thus is correlated with the height of the gap 36.

Another conventional air gauge that detects changes in air pressure is shown in FIG. 2, including an air supply 102, a regulator 104, a flow meter 106, a variable flow restrictor 108, and a junction 110 of a measurement branch 112 and a reference branch 114. The measurement branch 112 comprises a measurement conduit 116, a flow restrictor 118, and a probe 120. The reference branch 114 comprises a reference conduit 122, a constant-bleed flow restrictor 124, and a variable-bleed flow restrictor 126 that controllably discharges air to atmosphere. A bridge circuit 128 is connected to (and between) the measurement branch 112 and reference branch 114. Specifically, a first portion of the bridge circuit 128 is connected between the flow restrictor 118 and the probe 120, and a second end of the bridge circuit 128 is connected between the constant-bleed flow restrictor 124 and the variable-bleed flow restrictor 126. The bridge circuit 128 also includes a differential pressure (DP) sensor 130. The DP sensor 130 is connected between a first porous flow restrictor 132 (on the measurement side 112 of the bridge circuit 128) and a second porous flow restrictor 134 (on the reference side 114 of the bridge circuit 128).

The flow restrictor 118 imposes a resistance to flow of air through it. The resulting flow resistance produces a pressure drop across the flow restrictor, which is used to stabilize flow downstream of the flow restrictor. The porous flow restrictors 132, 134 generally have pressure drops of substantially zero, and act as low-pass filters to filter out high-frequency noise. The flow restrictors 118, 124, combined with the probe 120 and the variable-bleed flow restrictor 126, establish static conditions at the differential pressure sensor 130, wherein the differential pressure is substantially zero in the middle of the operational range of distances of the probe from the substrate surface 136. The variable-bleed flow restrictor 126 is adjusted appropriately to achieve the desired static condition.

The air gauges summarized above could be operated with either a mass-flow sensor or a differential pressure sensor. However, for applications requiring significant changes in height, where the differential pressure and/or the mass flow change appreciably, there is some advantage to measuring differential pressure rather than mass flow. The reason is that, in the mass-flow sensor case, the changing mass flow through the bridge conduit will affect the flow and pressure on the reference side of the air gauge. With conditions changing on both the measurement and reference sides, it becomes more difficult to operate the air gauge accurately. Using a differential pressure sensor in the bridge circuit 128 isolates the measurement and reference sides, making conditions on the reference side more stable.

For data processing, the data output from a differential pressure (DP) sensor (usually in analog form), is normally amplified and converted to corresponding digital data using an analog-to-digital converter (ADC). The ability of an air gauge to measure height with high accuracy requires a pressure sensor that produces low noise and provides high-resolution measurements. The resolution with which pressure measurements are obtained is set by the total pressure range achieved by the DP sensor during operation and by the number of bits in the ADC. Air gauges typically have very small operational ranges, 50 μιη for example. This requires that at least the measurement probe of the air gauge be proximal to the object within the operational range of 0 to 50 μιη (0 μιη would be actual contact). A concern with use of air gauges for determining height of fragile objects is the risk of collision of the air-gauge probe with the object, which could damage the object. One way to address this issue is to attach the air gauge to an actuator that controllably moves the gauge probe farther from the object when the gauge is not being used. A disadvantage of this approach is that this motion of the gauge is normally not servo-controlled. (Servo-control would require a separate sensor associated with the gauge or with the object as well as a separate actuator.)

Summary

Applicants have addressed this issue by, inter alia, configuring a DP air gauge with an extended operational range so as to avoid the complexity of having to servo the position of the air gauge relative to the object having a height to be measured by the air gauge. While taking this approach, applicants also satisfactorily addressed certain issues that otherwise would have prevented the air gauge from operating satisfactorily, for example, providing the air gauge with increased dynamic range without it producing excessive noise.

One aspect of the invention is directed to position-sensing devices, of which an embodiment comprises first and second gas passageways that are connectable to a source of pressurized gas. A "gas passageway" is any structure that can conduct flow of a gas to where the gas is needed or utilized. Hence, a gas passageway can be in the nature of a tube or other conduit. Gas passageways are not limited to tubes and conduits. For example, the gas passageway may be a void in an existing structure or may be a channel or gap. A gas-releasing probe is connected to the first passageway and is positionable relative to a surface and configured to release a first flow of the gas, flowing through the first passageway, toward the surface.

Meanwhile, a second flow of gas flows through the second passageway. A differential-pressure (DP) sensing device is connected between the first and second gas passageways. The DP-sensing device comprises first and second DP sensors that are connected together in parallel. The first DP sensor produces a gas-pressure measurement within a first dynamic -pressure range, and the second DP sensor produces a gas-pressure measurement within a second dynamic -pressure range that is larger than the first dynamic -pressure range. This DP-sensing device provides a position measurement having greater dynamic range and higher resolution than would otherwise be produced if the DP-sensing device comprised either the first or the second DP sensor alone.

Certain embodiments of the position-sensing devices can further comprise respective amplifiers connected to the first and second DP sensors. Respective analog-to-digital converters (ADCs) are connected to the amplifiers and to a controller so as to facilitate input of data to the controller from the respective DP sensors. The controller produces a controlled output based on the input data. The controlled output pertains, at least in part, to an actual position of the surface.

In some embodiments the first and second gas passageways comprise respective flow restrictors. Certain embodiments further comprise a bridging gas passageway that is connected to the first and second gas passageways and that conducts a respective flow of the gas between the first and second gas passageways. The bridging gas pathway can comprise respective flow restrictors connected to the ends of the DP-sensing device. The DP-sensing device can be connected to the bridging gas passageway so that the DP-sensing device senses differential pressure in the bridging gas passageway.

The first gas passageway can be configured to controllably bleed a flow of the gas to outside the device. To such end the second gas passageway can further comprise a variable-bleed flow restrictor located downstream of the second passageway's connection to the DP-sensing device.

The device can further comprise respective amplifiers connected to the first and second DP sensors, respective analog-to-digital converters (ADCs) connected to each amplifier, and a controller to which each of the ADCs is connected to input data from the respective ADCs into the controller. The controller produces a controlled output based on the input data, wherein the controlled output pertains, at least in part, to an actual position of the surface. According to another aspect of the invention, air gauges are provided that comprise first and second gas conduits connectable to a source of pressurized gas. A bridging conduit connects together the first and second gas conduit. First and second differential pressure (DP) measurement devices are connected together in parallel into the bridging conduit. The first measurement device produces DP data within a relatively low DP range, and the second measurement device produces DP data within a relatively high DP range so as to produce a position measurement having greater dynamic range and higher resolution than would otherwise be produced if the DP sensing device comprised either the first or the second DP- measurement device .

In another embodiment the air-gauge architecture is substantially identical, but instead of a source of high-pressure air or other gas, a vacuum is provided that draws air into the air gauge through the probe. The sensitivity of the differential pressure to changes in gap is similar to other embodiments.

According to yet another aspect of the invention, devices are provided for measuring distance to a measurement surface. An embodiment of such a device comprises a measurement portion comprising a respective measurement gas passageway and including a gas-discharging probe connected thereto. A reference portion comprises a respective reference gas passageway including a gas-bleed device. A supply conduit delivers gas from a gas source to the measurement and reference portions to produce respective flows of gas released from the measurement conduit by the probe onto a surface and released from the reference conduit by the gas-bleed device. A differential-pressure (DP) sensing device can be connected between the measurement portion and the reference portion. The DP sensing device comprising first and second DP sensors connected together in parallel. The first and second DP sensors collectively provide the DP sensing device with a greater dynamic range and greater resolution than would be provided if the DP sensing device comprised either the first or second DP sensor alone.

The first DP sensor can be configured for relatively high-resolution, relatively low-noise operation within a relatively low dynamic -pressure range.

Meanwhile the second DP sensor is configured for operation within a relatively high dynamic -pressure range. The device can further comprise a controller to which the DP sensors are connected so as to input data from the respective DP sensors into the controller. The controller produces a controlled output based on the input data. The controlled output pertains, at least in part, to an actual position of the surface. The device can further comprise a respective amplifier connected to each of the first and second DP sensors, and a respective analog-to-digital converter (ADC) connected to each amplifier, wherein the ADCs having respective outputs connected as inputs to the controller.

In a method for measuring a distance between a measurement surface and a measurement probe of an air gauge having a DP sensing device located between a measurement portion and a reference portion of the air gauge, another aspect of the invention pertains to improvements, in which the DP-sensing device is provided with a first DP sensor and a second DP sensor connected together in parallel. The first and second DP sensors, as connected, can be configured to provide the DP- sensing device with a greater dynamic range and greater resolution than otherwise would be provided if the DP-sensing device comprised either the first or second DP sensor alone.

According to yet another aspect of the invention, methods are provided for measuring distance from a measurement surface. In an embodiment of such a method, a controlled flow of gas is split at a first junction, resulting in a controlled flow being between a measurement pneumatic passageway and a reference pneumatic passageway. Gas flow in the measurement pneumatic passageway is directed through a measurement probe situated relative to the measurement surface. In a bridge passageway connected to the measurement pneumatic passageway and reference pneumatic passageway downstream of the first junction, differential pressure is sensed between the measurement pneumatic passageway and reference pneumatic passageway using a DP-sensing device. The DP-sensing device comprises a first DP sensor and a second DP sensor connected together in parallel. The first and second DP sensors collectively provide the DP-sensing device with greater dynamic range and resolution than otherwise would be provided if the DP- sensing device comprised either the first or second DP sensor alone. The foregoing and other features and advantages of the invention will be more apparent from the following detailed description, which proceeds with reference to the accompanying drawings. Brief Description of the Drawings

FIG. 1 is a schematic diagram of a conventional air gauge utilizing a backpressure bleed for reference purposes.

FIG. 2 is a schematic diagram of a conventional air gauge utilizing a differential pressure (DP) sensor.

FIG. 3 is a schematic diagram of a DP air gauge according to a

representative embodiment.

FIG. 4A is a plot of differential pressure (in units of Pascals) as a function of gap (in units of μιη) between the probe and the wafer surface, as exhibited by the conventional air gauge of FIG. 2.

FIG. 4B is a plot of sensitivity (Pa/μιη ) as a function of gap, as exhibited by the conventional air gauge of FIG. 2.

FIGS. 5A-5B are corresponding power spectra obtained during the 100- second tests of six particular DP sensors in the air gauge of FIG. 2.

FIGS. 6A-6B are corresponding power spectra obtained during the 600- second tests of six particular DP sensors in the air gauge of FIG. 2.

FIG. 7 is a schematic diagram of an embodiment of the air gauge that is an alternative to the embodiment of FIG. 3; only the region in the vicinity of the DP sensors is shown.

FIG. 8 is a schematic diagram of a microlithographic exposure system, as a representative precision system, including features of the invention described herein.

FIG. 9 is a flow-chart outlining a process for manufacturing a semiconductor device in accordance with the invention.

FIG. 10 is a flow-chart of a portion of a device-manufacturing process in more detail. Description

The invention is described below in the context of multiple exemplary embodiments, which are not intended to be limiting in any way.

The drawings are intended to illustrate the general manner of construction and are not necessarily to scale. In the detailed description and in the drawings themselves, specific illustrative examples are shown and described herein in detail. It will be understood, however, that the drawings and the detailed description are not intended to limit the invention to the particular forms disclosed, but are merely illustrative and intended to teach one of ordinary skill how to make and/or use the invention claimed herein.

As used in this application and in the claims, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. Additionally, the term "includes" means "comprises." Further, as applicable, the term "coupled" encompasses hydraulic, pneumatic, mechanical, electrical, as well as other practical ways of coupling or linking items together, and does not exclude the presence of intermediate elements between the coupled items.

The described things and methods described herein should not be construed as being limiting in any way. Instead, this disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed things and methods are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed things and methods require that any one or more specific advantages be present or problems be solved.

Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently.

Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed things and methods can be used in conjunction with other things and method. Additionally, the description sometimes uses terms like "produce" and "provide" to describe the disclosed methods. These terms are high- level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art.

In the following description, certain terms may be used such as "up," "down," "upper," "lower," "horizontal," "vertical," "left," "right," and the like.

These terms are used, where applicable, to provide some clarity of description when dealing with relative relationships. But, these terms are not intended to imply absolute relationships, positions, and/or orientations. For example, with respect to an object, an "upper" surface can become a "lower" surface simply by turning the object over. Nevertheless, it is still the same object.

During operation of a differential-pressure (DP) air gauge used to measure object height, it is useful for the air gauge to sense the object at larger distances than used for normal operation, so as to assist in avoiding collision of the object with the gauge. This capability requires pressure sensors having extended dynamic range. Unfortunately, such sensors typically produce more noise and have lower resolution, unless the DP sensor is connected to an analog-to-digital converter (ADC) having a larger number of bits. To satisfy both of these somewhat contradictory

requirements, as described herein, the subject air gauges comprise two or more DP sensors connected in parallel. I.e. , multiple DP sensors are utilized rather than only a single DP sensor as used in conventional air gauges. A first DP sensor is optimal for providing high-resolution, small-dynamic-range measurements at low noise within a defined limited dynamic pressure range. A second DP sensor has a larger dynamic range than the first DP sensor. The first DP sensor can utilize an ADC having the conventional number of bits {e.g. , 16 bits). Also, since the second DP sensor is not used for obtaining precise wafer-height measurements, it also does not need an ADC with more bits than normal. Thus, among various benefits, high costs are avoided by not having to select and utilize ADCs with more bits. Also, since ADCs with higher numbers of bits tend to operate more slowly than ADCs with lower numbers of bits, the speed at which the air gauge can obtain height measurements is not compromised relative to conventional air gauges.

A representative embodiment of an air gauge 200 comprising multiple DP sensors is shown in FIG. 3. The depicted embodiment 200 includes an air supply 202 (providing, for example, 400 kPaG supply of air), a regulator 204, an optional flow meter 206, an optional variable-flow restrictor 208, and a junction 210 of a measurement branch 212 and a reference branch 214. The measurement branch 212 comprises a measurement conduit 216, a flow restrictor 218, and a measurement probe 220. The reference branch 214 comprises a reference conduit 222, a constant- bleed flow restrictor 224, and a variable-bleed flow restrictor 226 that discharges air to atmosphere. A bridge circuit 228 is connected to (and between) the measurement branch 212 and reference branch 214. Specifically, a first portion of the bridge circuit 228 is connected between the flow restrictor 218 and the probe 220, and a second end of the bridge circuit 228 is connected between the constant-bleed flow restrictor 224 and the variable-bleed flow restrictor 226. The probe 220 is configured to be positionable at a convenient height relative to a surface 236, of a substrate, by any of various mechanisms (for convenient illustration, such mechanisms are not shown in FIG. 3). The bridge circuit 228 also includes a first differential pressure (DP) sensor 230a and a second DP sensor 230b. The DP sensors 230a, 230b are connected in parallel to each other in the bridge circuit 228 at junctions 231, 233. Optionally, the bridge circuit 228 can include a first flow restrictor 232 (on the measurement side 212 of the bridge circuit 228) and a second flow restrictor 234 (on the reference side 214 of the bridge circuit 228). Each DP sensor 230a, 230b is connected as an input to a respective amplifier 240a, 240b. Amplifier outputs are connected to respective analog-to-digital converters (ADC) 242a, 242b, which are connected to a controller 244. From data provided by the ADCs 242a, 242b, the controller 244 produces a controlled output that can be based on data from one or both of the DP sensors 230a, 230b. The controlled output can be, for example, data regarding an actual distance of a surface 236 from the probe 220. Based on this data, the height or relative height associated with the surface 236 can be selected to adjust focus for example. In some examples, instead of using two DP sensors, one having a larger dynamic range than the other, an optical sensor can be used. The probe 220 can be moved to a predetermined height with respect to the surface 236, and subsequent height measurements can then be provided with a DP sensor with high accuracy.

Exemplary flow restrictors include porous material, small apertures (or orifices), needle valves, and capillaries. Each of these types has slightly different properties from the other.

In work leading to conception of the embodiment 200 (FIG. 3), the conventional air gauge 100 of FIG. 2 was tested over an extended range of substrate height, using DP sensors manufactured by Freescale Semiconductor, Inc., Austin, TX. The normal range of operating height of the air gauge to the substrate surface is 50 + 5 μιη. Substrate height was measured at various DP values, up to a DP level corresponding to a maximum height of 105 μιη.

The air gauge 100 produced data regarding the respective heights of the substrate surface by measuring the difference of the corresponding internal pressures P_A and P_B at the ports p_l and p_2, respectively, using the DP sensor 130. From differential pressure determinations based on this data, changes in internal pressure accompanying changes in substrate height (more specifically, changes in the gap between the probe and the substrate surface) were determined. Hence, the substrate height (gap) determined the DP value.

For small changes in substrate height, the pressure change (dDP) is converted to a height change (dh) utilizing the particular sensitivity (S) of the air gauge:

S≡ dDP/dh

(1)

Consequently, dh = dDP/S

(2) in which S depends on both the probe design and the operation of the air gauge. A typical value of S is approximately 1 Pa/nm, occurring at a gap of 50 μιη. The sensitivity S tends to decrease with corresponding increases in gap height. In practice, it is desirable that S > 1 Pa/nm for good signal relative to noise.

Consequently, for determining wafer height in a microlithography system, a resolution of a fraction of a Pascal is required.

It is advantageous that the air gauge also be able to sense the wafer at larger distances than used for normal operation, to assist in avoiding collision of the probe of the gauge with the wafer. According to conventional thinking, increasing the range of the gap is achieved simply by using a DP sensor having a correspondingly extended dynamic range. But, since resolution is determined at least in part by the dynamic range, increasing the dynamic range of a DP sensor tends to reduce its resolution and increase its production of noise. Meanwhile, attainment of high measurement accuracy requires a DP sensor having low noise and high-resolution. Under some conditions, connecting the DP sensor to an ADC having a larger number of bits can reduce noise and increase resolution.

Exemplary test data are shown in FIGS. 4A and 4B. FIG. 4A is a plot of differential pressure (in units of Pascals) as a function of gap (in units of μιη) between the probe and the wafer surface. The gap ranged from 45 to 105 μιη in these experiments. Both curves were obtained using the same differential pressure sensor (Freescale model MPX5050. The label "FSDP" on the vertical axis is an abbreviation for Freescale DP sensor). At a gap of about 55 μιη, the variable-bleed flow restrictor was adjusted to keep the DP signal positive at 105 μιη. This resulted in a shift upward of the curve for gaps ranging from 50 to 105 μιη. The Freescale MPX5050 sensor has a pressure range of 0-50 kPa. It cannot read a negative differential pressure; therefore, the measurements were not carried out under the desired "balanced" condition in which the differential pressure is approximately zero in the middle of the operational height range. Other differential pressure sensors are bipolar and can be operated with the balanced condition. FIG. 4B is a plot of sensitivity as a function of gap. Sensitivity is in units of Pa/μιη. Both curves were obtained using the same differential pressure sensor (Freescale). The right-hand curve reveals a tendency of the sensitivity data to plateau rather than continue downward with increased gap. This result indicates that a good signal can be obtained even with larger gaps. Note that the relation between this DP sensor and the gap changed due to the earlier adjustments made to the flow controller to keep the FSDP signal positive at 105 μιη.

The tests also revealed that pressure-sensor noise increases with pressure range, which can limit the accuracy of the pressure measurements and hence of the gap measurements. Most of the low-frequency noise in the data came from the device configuration shown in FIG. 2. Hence, the desirability of comparing the performances of various pressure sensors themselves. Four differential pressure sensors were tested: MPX5010, MPX5050, MPX7007, and MPX7025 all manufactured by Freescale. Also tested were a differential pressure sensor (5 PSI- 4V-MINI) manufactured by All Sensors, Morgan Hill, CA, and a differential pressure sensor (model MB-LPS2-01) manufactured by Microbridge, Montreal, Canada. The range specifications for these sensors are listed below:

Sensor Pressure Range (kPa)

Freescale MPX5010 0-10 kPa

Freescale MPX5050 0-50 kPa

Freescale MPX7007 -7 to 7 kPa

Freescale MPX7025 -25 to 25 kPa

All Sensors 5 PSI-4V-MINI -35 to 35 kPa

Microbridge MB-LPS2-01 -1.5 to 1.5 kPa

During testing, the Freescale, Microbridge, and All Sensors pressure sensors were opened to air. Any atmospheric pressure perturbations would then be expected to be common mode, leading to no differential pressure at the sensor. Consequently, any noise produced by the pressure sensor was attributable to noise generated intrinsically (i.e., electronic noise). Data were taken for a period of 100 and 600 seconds. The tests were repeated with the sensor ports sealed for comparison, so as to provide control data.

FIGS. 5A-5B are power spectra obtained during the 100-second tests of all six differential pressure sensors. In FIG. 5A, note that noise per frequency is greater on the left rather than on the right. FIG. 5B contains integrated power spectrum plots for all six sensors. FIGS. 6A-6B are corresponding power spectra obtained during the 600-second tests of all six differential pressure sensors. The data show that the Microbridge sensors have the lowest noise of all the pressure sensors that were tested; but, because of their limited DP-measurement range (3 kPa), the Microbridge sensors have very limited applicability when used alone. Conversely, the All Sensors device had the highest DP-measurement range (70 kPa) but also had the highest noise level. Noise levels appear to be directly correlated to the DP ranges of the sensors.

The normal operating range of an air gauge in a lithography system is approximately +5 μιη. The desired resolution is 0.1 nm. Resolution is not exactly the same as the lowest significant bit (LSB) of the ADC connected to the differential pressure sensor. This is because averaging can provide a number that is more precise than 1 LSB. Consequently, an LSB of 0.1 Pa in a 16-bit ADC may be adequate even for sensitivities > 1 Pa/nm for the normal operating range. For extended-range operation, however, a DP sensor with extended range (and higher noise) and a slower, more expensive 18- or 20-bit ADC would be required.

Extended-range operation could be accomplished using a 16-bit ADC if a coarser LSB were used; but, both of these options degrade the gauge performance within the normal operating range. Test results are given in Table 1 and Table 2, setting forth data for LSB = 0.01 Pa and 1.0 Pa, respectively. Data for LSB = 0.01 Pa:

S = 0.5 S = 1 S = 2

ADC ADC Dyn. Range Height Range Height Range Height Range

Bits Range ± (Pa) + (nm) + (nm) + (nm)

12 4096 204.8 409.6 204.8 102.4

16 65536 3276.8 6553.6 3276.8 1638.4

18 262144 13107.2 26214.4 13107.2 6553.6

20 1048576 52428.8 104857.6 52428.8 26214.4

Table 2

Data for LSB = 1 Pa:

S = 0.5 S = 1 S = 2

ADC ADC Dyn. Range Height Range Height Range Height Range

Bits Range ± (Pa) + (nm) + (nm) + (nm)

12 4096 2048 4096 2048 1024

16 65536 32768 65536 32768 16384

18 262144 131072 262144 131072 65536

20 1048576 524288 1048576 524288 262144

Note that S = 2 Pa/nm is more sensitive than S = 0.5 Pa/nm. Hence, increased sensitivity yields decreased height range. From these data, 16-bit ADCs appeared to be adequate for extended-range operation.

From the testing described above, a conception was realized in which, to provide optimal operation in both normal and extended operating ranges, two DP sensors would be used. The two DP sensors are connected together in parallel and utilized in place of a single DP sensor used in the conventional air gauge shown in FIG. 2. An embodiment of the multiple-DP sensor air gauge is shown in FIG. 3, in which DPI (item 230a) is a small-range DP sensor that produces low noise and utilizes a small-LSB ADC for high resolution, and DP2 (item 230b) is a larger-range DP sensor utilizing a coarser-LSB ADC for extended range. Both DP sensors 230a, 230b utilize a respective 16-bit ADC 242a, 24b, as noted above. Amplifiers 240a, 240b are employed for adjusting the differential pressure changes to the voltage ranges of the ADCs 242a, 242b, respectively.

The flow restrictor 218 is configured to impose respective defined resistances to flow of air through it. The resulting resistance to air flow, while producing a pressure drop across the flow restrictor, is used to stabilize flow downstream of the flow restrictor. The flow restrictors 232, 234 generally have pressure drops of substantially zero, and act as low-pass filters to filter out high- frequency noise. The flow restrictors 218, 224, combined with the probe 220 and the variable-bleed flow restrictor 226, establish static conditions at the differential pressure sensors 230a, 230b, wherein the differential pressure is substantially zero in the middle of the operational range of distances of the probe 220 from the substrate surface 236. The variable-bleed flow restrictor 226 is adjusted appropriately to achieve the desired static condition.

In FIG. 3 the flow restrictors 232,234 are located before the junctions separating the DP sensors 230a, 230b. Another embodiment of this geometry is shown in FIG. 7, in which the flow restrictors are 332a, 332b associated with a measurement path 312 are located adjacent the DP sensors 330a, 330b on their respective measurement sides. Flow restrictors 334a, 334b associated with a reference path 314 are also located adjacent the DP sensors 330a, 330b on their respective reference sides. In this configuration, the flow restrictors 332a, 332b, 334a, 334b separate the DP sensors 330a, 330b from the junctions 331, 333 to provide enhanced isolation for the DP sensors 330a, 330b. The DP sensors 330a, 330b are coupled via respective amplifiers 340a, 340b to analog-to-digital convertors and a control system in an arrangement similar to that of FIG. 3. In the configuration of FIG. 3, a pressure surge that is partially transmitted by the flow restrictors 232, 234 can divide at the junctions 231, 233 and reach the DP sensors 230a, 230b. Some of the pressure surge can be reflected at the first DP sensor 230a and then travel to the second DP sensor 230b, interfering with operation of the sensors 230a, 230b. In the example of FIG. 7, there is no path between the DP sensors 330a, 330b that is not filtered by flow restrictors. In an alternative embodiment the DP sensors have substantially identical properties but each is adjusted so that only a fraction of the sensor' s dynamic range spans the range of the ADC to which the DP sensor is connected, thereby providing improved resolution.

Although the devices disclosed herein are termed "air" gauges, it will be understood that any of various gases other than air can be used.

Included in this disclosure are any of various precision systems comprising a stage or the like that holds a workpiece or other item useful in a manufacture, relative to an axis, and that determines location of the stage at high accuracy and precision using devices and methods as described above. An example of a precision system is a microlithography system or exposure "tool" used for manufacturing semiconductor devices. A schematic depiction of an exemplary microlithography system 810, comprising features of the invention described herein, is provided in FIG. 8. The system 810 includes a system frame 812, an illumination system 814, an imaging-optical system 816, a reticle-stage assembly 818, a substrate- stage assembly 820 that includes a position/sensor system 820A, a positioning system 822, and a system-controller 824. The configuration of the components of the system 810 is particularly useful for transferring a pattern (not shown) of an integrated circuit from a reticle 826 onto a semiconductor wafer 828. The height of the semiconductor wafer can be determined using air gauge systems as described above. The system 810 mounts to a mounting base 830, e.g. , the ground, a base, or floor or other supporting structure. At least one of the stage assemblies 818, 820 includes a linear or planar motor comprising coil modules and at least one coil assembly as disclosed above.

An exemplary process for manufacturing semiconductor devices, including an exposure step, is shown in FIG. 9. In step 901 the device's function and performance characteristics are designed. Next, in step 902, a mask (reticle) having a desired pattern is designed according to the previous designing step, and in a parallel step 903 a wafer is made from a suitable semiconductor material. The mask pattern designed in step 902 is exposed onto the wafer from step 903 in step 904 by a microlithography system described herein in accordance with the present invention. Adjustments based on air gauge systems can be made in step 910. In step 905 the semiconductor device is assembled (including the dicing process, bonding process, and packaging process). Finally, the device is inspected in step 906.

FIG. 10 is a flowchart of the above-mentioned step 1004 in the case of fabricating semiconductor devices. In FIG. 10, in step 1011 (oxidation step), the wafer surface is oxidized. In step 1012 (CVD step), an insulation film is formed on the wafer surface. In step 1013 (electrode-formation step), electrodes are formed on the wafer by vapor deposition. In step 1014 (ion-implantation step), ions are implanted in the wafer. The above-mentioned steps 1011-1014 constitute the preprocessing steps for wafers during wafer processing, and selection is made at each step according to processing requirements.

At each stage of wafer-processing, when the preprocessing steps have been completed, the following "post-processing" steps are implemented. During post- processing, first, in step 1015 (photoresist-formation step), photoresist is applied to a wafer. Next, in step 1016 (exposure step), the above-mentioned exposure device is used to transfer the circuit pattern of a mask (reticle) to a wafer. Sensing systems such as the dual air gauge systems described above can be used for position sensing. Then, in step 1017 (developing step), the exposed wafer is developed, and in step 1018 (etching step), parts other than residual photoresist (exposed material surface) are removed by etching. In step 1019 (photoresist-removal step), unnecessary photoresist remaining after etching is removed. Multiple circuit patterns are formed by repetition of these pre-processing and post-processing steps.

The devices disclosed herein can be used readily in any of various precision systems to determine height (or more generally position) of a component in the system or an object manipulated or worked by the system. Typically, an air gauge is mounted to, or otherwise placed relative to, structure having high stability and a fixed location, desirably free of vibrations. For example, an air gauge can be mounted to the metrology frame of a microlithography system, or mounted to a lens assembly of a microlithography system, or mounted at a fixed, stable location relative to the lens assembly. In a microlithography system the air gauge can be a portion of an autofocus (AF) device used, for example, for determining substrate height relative to an imaging optical system.

Claims

What is claimed is:

1. A position-sensing device, comprising:

first and second gas passageways connectable to a source of pressurized gas; a gas-releasing probe connected to the first passageway, the probe being positionable relative to a surface and configured to release a first flow of the gas, flowing through the first passageway, toward the surface while a second flow of the gas flows through the second passageway; and

a differential-pressure (DP) sensing device connected between the first and second passageways and comprising first and second DP sensors connected together in parallel, wherein the first DP sensor produces a gas-pressure measurement within a first dynamic-pressure range, and the second DP sensor produces a gas-pressure measurement in a second dynamic-pressure range larger than the first dynamic pressure range.

2. The device of claim 1, wherein the DP sensing device produces a position measurement having greater dynamic range and higher resolution than would otherwise be produced if the DP sensing device comprised either the first DP sensor or the second DP sensor alone.

3. The device of claim 1, further comprising:

respective amplifiers connected to the first and second DP sensors;

respective analog-to-digital converters (ADCs) connected to the amplifiers; and

a controller to which the ADCs are connected to input data from the respective ADCs into the controller, the controller producing a controlled output based on the input data.

4. The device of claim 1, wherein the controlled output pertains, at least in part, to an actual position of the surface.

5. The device of claim 1, wherein the first and second gas passageways comprise respective flow restrictors.

6. The device of claim 1, further comprising a bridging gas passageway connected to the first and second gas passageways and conducting a respective flow of the gas between the first and second gas passageways.

7. The device of claim 6, wherein the DP-sensing device is connected to the bridging gas passageway so that the DP-sensing device senses differential pressure in the bridging gas passageway.

8. The device of claim 7, wherein the bridging gas passageway comprises a respective flow restrictor connected to each end of the DP-sensing device.

9. The device of claim 8, wherein the first and second gas passageways comprise respective flow restrictors.

10. The device of claim 1, wherein the first gas passageway is configured to controllably bleed a flow of the gas to outside the device.

11. The device of claim 10, wherein the second gas passageway further comprises a variable bleed flow restrictor located downstream of the second passageway's connection to the DP-sensing device.

12. The device of claim 11, further comprising:

respective amplifiers connected to the first and second DP sensors;

respective analog-to-digital converters (ADCs) connected to each amplifier; and a controller to which the ADCs are connected to input data from the respective ADCs into the controller, the controller producing a controlled output based on the input data, the controlled output pertaining, at least in part, to an actual position of the surface.

13. The device of claim 1, wherein the DP- sensing device is connected across a bridging gas passageway extending between and connecting together the first and second gas passageways.

14. An air gauge, comprising:

first and second gas conduits connectable to a source of pressurized gas; a bridging conduit connecting together the first and second gas conduits; and first and second differential pressure (DP) measurement devices connected together in parallel into the bridging conduit;

wherein the first measurement device produces DP data within a first DP range, and the second measurement device produces DP data within a second DP range larger than the first DP range.

15. The air gauge of claim 14, wherein the first and second DP measurement devices produce a position measurement having greater dynamic rang and higher resolution than would otherwise be produced if the DP sensing device comprised either the first or the second DP sensor.

16. A precision system, comprising a position-measurement device recited in claim 1.

The precision system of claim 16, configured as a microlithography

A precision system, comprising an air gauge as recited in claim 14.

19. The precision system of claim 18, configured as a microlithography system.

20. A device for measuring distance to a measurement surface, the device comprising:

a measurement portion comprising a respective measurement gas

passageway and including a gas-discharging probe connected thereto;

a reference portion comprising a respective reference gas passageway including a gas-bleed device;

a supply conduit delivering gas from a gas source to the measurement and reference portions to produce respective flows of gas released from the measurement conduit by the probe onto a surface and released from the reference conduit by the gas -bleed device; and

a differential-pressure (DP) sensing device connected between the measurement portion and the reference portion, the DP sensing device comprising first and second DP sensors connected together in parallel.

21. The device of claim 20, wherein the first and second DP sensors collectively provide the DP sensing device with a greater dynamic range and greater resolution than would be provided if the DP sensing device comprised either the first or second DP sensor alone.

22. The device of claim 21, wherein the first DP sensor is configured for relatively high-resolution, relatively low-noise operation within a relatively low dynamic -pressure range, and the second DP sensor is configured for operation within a relatively high dynamic-pressure range.

23. The device of claim 21, further comprising a controller to which the DP sensors are connected to input data from the respective DP sensors into the controller, the controller producing a controlled output based on the input data, the controlled output pertaining, at least in part, to an actual position of the surface.

24. The device of claim 23, further comprising:

a respective amplifier connected to each of the first and second DP sensors; and

a respective analog-to-digital converter (ADC) connected to each amplifier, the ADCs having respective outputs connected as inputs to the controller.

25. In an air gauge including a measurement conduit and a reference conduit pneumatically coupled together by a DP sensing device, an improvement in which the DP sensing device comprises a first DP sensor and a second DP sensor connected together in parallel.

26. The air gauge of claim 25, wherein the first and second DP sensors collectively provide the DP sensing device with a greater dynamic range and greater resolution than otherwise would be provided if the DP sensing device comprised either the first or second DP sensor alone.

27. The air gauge of claim 25, wherein the first DP sensor is configured for relatively high-resolution, relatively low-noise operation within a relatively low dynamic -pressure range, and the second DP sensor is configured for operation within a relatively high dynamic-pressure range.

28. In a method for measuring a distance between a measurement surface and a measurement probe of an air gauge having a DP- sensing device located between a measurement portion and a reference portion of the air gauge, an improvement comprising:

providing the DP-sensing device with a first DP sensor and a second DP sensor connected together in parallel; and

configuring the first and second DP sensors, as connected, collectively to provide the DP sensing device with a greater dynamic range and greater resolution than otherwise would be provided if the DP sensing device comprised either the first or second DP sensor alone.

29. The method of claim 28, further comprising configuring the first DP sensor for relatively high-resolution, relatively low-noise operation within a relatively low dynamic-pressure range, and configuring the second DP sensor for operation within a relatively high dynamic -pressure range.

30. A method for measuring distance from a measurement surface, the method comprising:

at a first junction, splitting a controlled flow of gas between a measurement pneumatic passageway and a reference pneumatic passageway;

directing gas flow in the measurement pneumatic passageway through a measurement probe situated relative to the measurement surface; and

in a bridge passageway connected to the measurement pneumatic

passageway and reference pneumatic passageway downstream of the first junction, sensing differential pressure between the measurement pneumatic passageway and reference pneumatic passageway using a DP-sensing device comprising a first DP sensor and a second DP sensor connected together in parallel.

31. The method of claim 30, wherein the first and second DP sensors collectively provide the DP-sensing device with greater dynamic range and resolution than otherwise would be provided if the DP-sensing device comprised either the first or second DP sensor alone.

32. A position-sensing device, comprising:

first and second gas passageways connectable to a source of pressurized gas; a gas-releasing probe connected to the first passageway, the probe being positionable relative to a surface and configured to release a first flow of the gas, flowing through the first passageway, toward the surface while a second flow of the gas flows through the second passageway; a first sensing device connected between the first and second passageways, the first sensing device comprising a first differential-pressure sensor that produces a gas-pressure measurement within a first dynamic-pressure range; and

a second sensing device that produces a position measurement in a second dynamic range that is larger than the first dynamic -pressure range.

33. The device of claim 32, wherein the second sensing device comprises an optical sensor.

34. The device of claim 32, wherein the second sensing device comprises a second differential-pressure sensor that produces a gas-pressure measurement within a second dynamic-pressure range.