Method and system for determining characteristics of substrates employing fluid geometries

BACKGROUND OF THE INVENTION

0001, The present invention relates generally to imprint lithography systems. More particularly, the present invention is directed to determining spatial relationships between an imprinting mold and a substrate upon which a pattern will be formed using the imprinting mold.

0002Imprint lithography has shown promising results in fabrication of patterns having feature sizes smaller than 50nm. As a result, many prior art imprint lithography techniques have been advocated. United States patent number <patcit id="pcit0001" dnum="US6334960B"><text>6,334,960 to Willson et al.</text></patcit> discloses an exemplary lithography imprint technique that includes providing a substrate having a transfer layer. The transfer layer is covered with a polymerizable fluid composition. A mold makes mechanical contact with the polymerizable fluid. The mold includes a relief structure, and the polymerizable fluid composition fills the relief structure. The polymerizable fluid composition is then subjected to conditions to solidify and polymerize the same, forming a solidified polymeric material on the transfer layer that contains a relief structure complimentary to that of the mold. The mold is then separated from the solid polymeric material such that a replica of the relief structure in the mold is formed in the solidified polymeric material. The transfer layer and the solidified polymeric material are subjected to an environment to selectively etch the transfer layer relative to the solidified polymeric material to form a relief image in the transfer layer.

0003United States patent number <patcit id="pcit0002" dnum="US5772905A"><text>5,772,905 to Chou</text></patcit> discloses an imprint lithographic method and apparatus for creating patterns in a thin film coated on a substrate in which a mold, having at least one protruding feature is pressed into a thin film carried on a substrate. The protruding feature in the mold creates a recess in the thin film. The mold is removed from the film. The thin film then is processed such that the thin film in the recess is removed exposing the underlying substrate. Thus, patterns in the mold are replaced in the thin film, completing the lithography. The patterns in the thin film will be, in subsequent processes, reproduced in the substrate or in another material which is added onto the substrate.

0004Yet another imprint lithography technique is disclosed by <nplcit id="ncit0001" npl-type="s"><text>Chou et al. in Ultrafast and Direct Imprint of Nanostructures in Silicon, Nature, Col. 417, pp. 835-837, June 2002</text></nplcit>, which is referred to as a laser assisted direct imprinting (LADI) process. In this process a region of a substrate is made flowable, e.g., liquefied, by heating the region with the laser. After the region has reached a desired viscosity, a mold, having a pattern thereon, is placed in contact with the region. The flowable region conforms to the profile of the pattern and is then cooled, solidifying the pattern into the substrate.

0005An important consideration when forming patterns in this manner is to maintain control of the distance and orientation between the substrate and the mold that contains the pattern to be recorded on the substrate. Otherwise, undesired film and pattern anomalies may occur.

0006There is a need, therefore, for accurately determining spatial relationships between a mold and a substrate upon which the mold will form a pattern using imprinting lithographic processes. (<patcit id="pcit0003" dnum="WO02067055A"><text>WO 02/067055</text></patcit> discloses a method to detect markings in order to align a template and a substrate. <patcit id="pcit0004" dnum="US5837892A"><text>US 5837892</text></patcit> discloses a vision system to locate fiducial points for aligning a dispenser with a circuit board. The vision system is also used, after dispensing, to verify volume and location of the dispensed fluid. Finally, the vision system is used during dispenser calibration to measure the volume of the dispensed drops.

SUMMARY OF THE INVENTION

0007The present invention provides a method and system according to the appended claims, of determining characteristics of substrates, such as the spatial relationships between spaced-apart substrates. The spatial relationships include distance and angular orientation, between first and second spaced apart substrates. The method includes forming a volume of fluid on the second substrate, with the volume of fluid having an area associated therewith. The volume of fluid is compressed between the first and second substrates to effectuate a change in properties of the area, defining changed properties. The changed properties are sensed, and the characteristics of the first and second substrates are determined as a function of the changed properties. The system includes features to carry-out the functions of the method. These and other embodiments are discussed more fully below.

BRIEF DESCRIPTION OF THE DRAWINGS

0008<figref idref="f0001">Fig. 1</figref> is a simplified plan view of an imprint lithographic system incorporating a detection system in accordance with one embodiment of the present invention;

0009<figref idref="f0002">Fig. 2</figref> is a partial simplified elevation view of an imprint lithographic system shown in <figref idref="f0001">Fig. 1</figref>;

0010<figref idref="f0002">Fig. 3</figref> is a simplified representation of material from which an imprinting layer, shown in <figref idref="f0002">Fig. 2</figref>, is comprised before being polymerized and cross-linked;

0011<figref idref="f0003">Fig. 4</figref> is a simplified representation of cross-linked polymer material into which the material shown in <figref idref="f0002">Fig. 3</figref>, is transformed after being subjected to radiation;

0012<figref idref="f0003">Fig. 5</figref> is a simplified elevation view of a mold spaced-apart from an imprinting layer, shown in <figref idref="f0001">Fig. 1</figref>, after patterning of the imprinting layer;

0013<figref idref="f0003">Fig. 6</figref> is a simplified elevation view of an additional imprinting layer positioned atop of the substrate, shown in <figref idref="f0003">Fig. 5</figref>, after the pattern in the first imprinting layer is transferred therein;

0014<figref idref="f0004">Fig. 7</figref> is a top down view of a region of a wafer, shown in <figref idref="f0001">Fig. 1</figref>, that is sensed by a detection system shown therein in accordance with one embodiment of the present invention;

0015<figref idref="f0004">Fig. 8</figref> is a cross-section of the resulting shape of an imprinting layer shown in <figref idref="f0001">Fig. 1</figref>, being formed with the mold and the wafer not being in parallel orientation with respect to one another;

0016<figref idref="f0004">Fig. 9</figref> is a top down view of a region of a wafer, shown in <figref idref="f0001">Fig. 1</figref>, that is sensed by a detection system shown therein in accordance with an alternate embodiment of the present invention;

0017<figref idref="f0005">Fig. 10</figref> is a top down view of a region of a wafer, shown in <figref idref="f0001">Fig. 1</figref>, that is sensed by a detection system shown therein in accordance with another alternate embodiment of the present invention;

0018<figref idref="f0005">Fig. 11</figref> is a simplified plan view of an imprint lithographic system incorporating a detection in accordance with a second embodiment of the present invention; and

0019<figref idref="f0006">Fig. 12</figref> is a simplified plan view of an imprint lithographic system incorporating a detection system in accordance with a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

0020<figref idref="f0001">Fig. 1</figref> depicts an imprint lithographic system 10 in which a detection system in accordance with one embodiment of the present invention is included. System 10 includes an imprint head 12 and a stage 14, disposed opposite to imprint head 12. A radiation source 16 is coupled to system 10 to impinge actinic radiation upon motion stage 14. To that end, imprint head 12 includes a throughway 18 and a mirror 20 couples actinic radiation from radiation source 16, into throughway 18 to impinge upon a region 22 of stage 14. Disposed opposite to region 22 is a detection system that includes a CCD sensor 23 and wave shaping optics 24. CCD sensor 23 is positioned to sense images from region 22. Detection system is configured with wave shaping optics 24 positioned between CCD sensor 23 and mirror 20. A processor 25 is in data communication with CCD sensor 23, imprint head 12, stage 14 and radiation source 16.

0021Referring to both <figref idref="f0001">Figs. 1</figref> and <figref idref="f0002">2</figref>, connected to imprint head 12 is a first substrate 26 having a mold 28 thereon. First substrate 26 may be held to imprint head 12 using any known technique. In the present example first substrate 26 is retained by imprint head 12 by use of a vacuum chuck (not shown) that is connected to imprint head 12 and applies a vacuum to first substrate 26. An exemplary chucking system that may be included is disclosed in United States patent application number <patcit id="pcit0005" dnum="US10293224B"><text>10/293,224</text></patcit> entitled "A Chucking System for Modulating Shapes of Substrates" . Mold 28 may be planar or include a feature thereon. In the present example, mold 28 includes a plurality of features defined by a plurality of spaced-apart recessions 28a and protrusions 28b. The plurality of features defines an original pattern that is to be transferred into a second substrate, such as wafer 30, coupled to stage 14. To that end, imprint head 12 is adapted to move along the Z axis and vary a distance "d" between mold 28 and wafer 30. Stage 14 is adapted to move wafer 30 along the X and Y axes, with the understanding that the Y axis is into the sheet upon which <figref idref="f0001">Fig. 1</figref> is shown. With this configuration, the features on mold 28 may be imprinted into a flowable region of wafer 30, discussed more fully below. Radiation source 16 is located so that mold 28 is positioned between radiation source 16 and wafer 30. As a results, mold 28 is fabricated from material that allows it to be substantially transparent to the radiation produced by radiation source 16, such as fused silica or quartz glass.

0022Referring to both <figref idref="f0002">Figs. 2 and 3</figref>, a flowable region, such as an imprinting layer 34, is disposed on a portion of surface 32 that presents a substantially planar profile. Flowable region may be formed using any known technique such as a hot embossing process disclosed in United States patent number <patcit id="pcit0006" dnum="US5772905A"><text>5,772,905</text></patcit>, or a laser assisted direct imprinting (LADI) process of the type described by <nplcit id="ncit0002" npl-type="s"><text>Chou et al. in Ultrafast and Direct Imprint of Nanostructures in Silicon, Nature, Col. 417, pp. 835-837, June 2002</text></nplcit>. In the present embodiment, however, flowable region consists of imprinting layer 34 being deposited as a plurality of spaced-apart discrete beads 36 of material 36a on wafer 30, discussed more fully below. Imprinting layer 34 is formed from a material 36a that may be selectively polymerized and cross-linked to record the original pattern therein, defining a recorded pattern. Material 36a is shown in <figref idref="f0003">Fig. 4</figref> as being cross-linked at points 36b, forming cross-linked polymer material 36c.

0023Referring to <figref idref="f0002">Figs. 2, 3</figref> and <figref idref="f0003">5</figref>, the pattern recorded in imprinting layer 34 is produced, in part, by mechanical contact with mold 28. To that end, imprint head 12 reduces the distance "d" to allow imprinting layer 34 to come into mechanical contact with mold 28, spreading beads 36 so as to form imprinting layer 34 with a contiguous formation of material 36a over surface 32. Were mold 28 provided with a planar surface, distance "d" would be reduced to provide imprinting layer 34 with a substantially planar surface. In the present example, distance "d" is reduced to allow sub-portions 34a of imprinting layer 34 to ingress into and fill recessions 28a.

0024To facilitate filling of recessions 28a, material 36a is provided with the requisite properties to completely fill recessions 28a while covering surface 32 with a contiguous formation of material 36a. In the present example, sub-portions 34b of imprinting layer 34 in superimposition with protrusions 28b remain after the desired, usually minimum distance "d", has been reached, leaving sub-portions 34a with a thickness t₁, and sub-portions 34b with a thickness, t₂. Thicknesses "t₁" and "t₂" may be any thickness desired, dependent upon the application. Typically, t₁ is selected so as to be no greater than twice the width u of sub-portions 34a, i.e., t₁ &lt; 2u, shown more clearly in <figref idref="f0003">Fig. 5</figref>.

0025Referring to <figref idref="f0002">Figs. 2, 3</figref> and <figref idref="f0003">4</figref>, after a desired distance "d" has been reached, radiation source 16, shown in <figref idref="f0001">Fig. 1</figref>, produces actinic radiation that polymerizes and cross-links material 36a, forming cross-linked polymer material 36c. As a result, the composition of imprinting layer 34, transforms from material 36a to material 36c, which is a solid. Specifically, material 36c is solidified to provide side 34c of imprinting layer 34 with a shape conforming to a shape of a surface 28c of mold 28, shown more clearly in <figref idref="f0003">Fig. 5</figref>. After imprinting layer 34 is transformed to consist of material 36c, shown in <figref idref="f0003">Fig. 4</figref>, imprint head 12, shown in <figref idref="f0002">Fig. 2</figref>, is moved to increase distance "d" so that mold 28 and imprinting layer 34 are spaced-apart.

0026Referring to <figref idref="f0003">Fig. 5</figref>, additional processing may be employed to complete the patterning of wafer 30. For example, wafer 30 and imprinting layer 34 may be etched to transfer the pattern of imprinting layer 34 into wafer 30, providing a patterned surface 32a, shown in <figref idref="f0003">Fig. 6</figref>. To facilitate etching, the material from which imprinting layer 34 is formed may be varied to define a relative etch rate with respect to wafer 30, as desired. The relative etch rate of imprinting layer 34 to wafer 30 may be in a range of about 1.5:1 to about 100:1.

0027Alternatively, or in addition to, imprinting layer 34 may be provided with an etch differential with respect to photo-resist material (not shown) selectively disposed thereon. The photo-resist material (not shown) may be provided to further pattern imprinting layer 34, using known techniques. Any etch process may be employed, dependent upon the etch rate desired and the underlying constituents that form wafer 30 and imprinting layer 34. Exemplary etch processes may include plasma etching, reactive ion etching, chemical wet etching and the like.

0028Referring to both <figref idref="f0001">Figs. 1</figref> and <figref idref="f0002">2</figref>, an exemplary radiation source 16 may produce ultraviolet radiation. Other radiation sources may be employed, such as thermal, electromagnetic and the like. The selection of radiation employed to initiate the polymerization of the material in imprinting layer 34 is known to one skilled in the art and typically depends on the specific application which is desired. Furthermore, the plurality of features on mold 28 are shown as recessions 28a extending along a direction parallel to protrusions 28b that provide a cross-section of mold.28 with a shape of a battlement. However, recessions 28a and protrusions 28b may correspond to virtually any feature required to create an integrated circuit and may be as small as a few tenths of nanometers. As a result, it may be desired to manufacture components of system 10 from materials that are thermally stable, e.g., have a thermal expansion coefficient of less than about 10 ppm/degree Centigrade at about room temperature (e.g. 25 degrees Centigrade). In some examples of application, the material of construction may have a thermal expansion coefficient of less than about 10 ppm/degree Centigrade, or less than 1 ppm/degree Centigrade.

0029Referring to <figref idref="f0001">Figs. 1</figref>, <figref idref="f0002">2</figref> and <figref idref="f0004">7</figref>, an important consideration to successfully practice imprint lithography techniques is accurately determining distance "d". To that end, the detection system of the present invention is configured to take advantage of the change in the geometry of beads 36 as the distance "d" is reduced. Assuming beads 36 behave as a non-compressible fluid with a volume "v", distance "d" may be defined as follows: <maths id="math0001" num="(1)"><math display="block"><mi>d</mi><mo>=</mo><msub><mmultiscripts><mo>/</mo><mprescripts /><none /><mi>v</mi></mmultiscripts><mi>A</mi></msub></math><img file="EP1570249B1_D0001.tif" /></maths> where A is a liquid filled area measured by CCD sensor 23. To that end, the combination of CCD sensor 23 and wave shaping optics 24 allows the detection system to sense one or more beads 36 in region 22. With first substrate 26 spaced apart from wafer 30, the volume of one or more beads 36 provides each bead 36 with an area 40 associated therewith. As distance "d" is reduced and substrate 26 comes into mechanical contact with beads 36, compression occurs. This compression effectuates a change in properties of the area 40 of beads 36, referred to as changed properties. These changes relate to the geometries of one or more beads 36, such as the shape, size or symmetry of the area. In the present example the changed properties are shown as 42 and concern the size of the area. Specifically, the compression results in the area of beads 36 increasing.

0030The change in area 40 is sensed by CCD sensor 23, which produces data corresponding to the same. Processor 25 receives the data corresponding to the change in the area 40 calculates, using equation 1, the distance "d". Assuming CCD sensor 23 consists of a N x M array of pixels, distance "d" is ascertained by processor 25 through the following equation: <maths id="math0002" num="(2)"><math display="block"><mi>d</mi><mo>=</mo><msub><mmultiscripts><mo>/</mo><mprescripts /><none /><mi>v</mi></mmultiscripts><msub><mi>t</mi><mi>p</mi></msub></msub><mfenced><msub><mi>P</mi><mi>a</mi></msub></mfenced></math><img file="EP1570249B1_D0002.tif" /></maths> where t_p is the total number of pixels in the N x M array and P_a is the area of each pixel.

0031With volume of beads 36 being fixed, the resolution of CCD sensor 23 that is desired to accurately measure the area A may be defined as follows: <maths id="math0003" num="(3)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>A</mi><mo>=</mo><mfenced><msub><mmultiscripts><mo>/</mo><mprescripts /><none /><mi>A</mi></mmultiscripts><mi>d</mi></msub></mfenced><mo>⁢</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>d</mi></math><img file="EP1570249B1_D0003.tif" /></maths> Assuming that the total volume, v, of one of beads 36 sensed by CCD sensor 23 is 200nl, i.e., 0.1 mm³ and d = 200nm, then liquid filled area "A" is 1000mm². From equation (2) it may be determined that the desired resolution of CCD sensor 23 is 5mm².

0032It should be noted that processor 25 may be employed in a feedback loop operation. In this manner, distance "d" may be calculated multiple times, until it is determined that the desired distance "d" has been reached. Such calculations may be performed dynamically in real time, or sequentially, with the distance "d" being determined as incremental movements of imprint head 12 along the Z axis occur. Alternatively, or in addition thereto, processor 25 may be in data communication with a memory 27 that includes computer-readable information in the form of a look-up table 29. The information in look-up table 29 may include geometries , shown as 31a, 31b and 31c as related to differing distances, shown as d_a, d_b and d_c. In this manner, information concerning the geometry of one or more beads 36 may be obtained by CCD sensor 23 and received by processor 25. The information is then processed to relate the same to the geometry in look-up table 29 that most closely matches the geometry of the one or more beads 36 sensed by CCD sensor 23. Once a match is made, processor determines a magnitude of distance d present in look-up table 29 that is associated with the matching geometry.

0033Additional information concerning characteristics of first substrate 26 and wafer 30 may be other than the distance d therebetween may be obtained by analyzing the fluid geometry of one or more beads 36. For example, by analyzing the symmetry of beads 36 an angular orientation between first substrate 26 and wafer 30 may be determined. Assume first substrate 26 lies in a first plane P₁ and wafer 30 lies in a second plane P₂. Assuming area 40 is radially symmetric, any loss of radial symmetry in area 40 may be employed to determine that first plane P₁ and second plane P₂ do not extend parallel to one another. Additionally, data concerning the shape of area 40, in this case the lack of radial symmetry, may be employed to determine the angle <img file="EP1570249B1_D0004.tif" /> formed between first and second planes P₁ and P₂ and, therefore, between first substrate 26 and wafer 30, shown in <figref idref="f0004">Fig. 8</figref>. As a result, undesired thicknesses in imprinting layer 34 may be ascertained and, therefore, avoided. Other information may be obtained, as well, such as the contamination of first substrate 26 or wafer 30 or both by particulate matter.

0034Specifically, the presence of particulate matter on substrate may manifest as many different shapes. For purposes of the present discussion, one or more beads 36 having an asymmetrical area associated therewith may indicate the presences of particulate contaminants on either first substrate 26 or wafer 30. Further, with a priori knowledge of contaminants, specific shapes of one ore more beads 36 may be associated with a particular defect, such as particulate contamination, as well as the presence of the defect, e.g., on first substrate 26, wafer 30 and/or stage. This information may be included in a look-up table as discussed above so that processor may classify the defect and characterize first substrate 26 and/or wafer 30, accordingly.

0035Referring to <figref idref="f0001">Figs. 1</figref>, <figref idref="f0002">2</figref> and <figref idref="f0004">9</figref>, by analyzing information from two or more beads, shown as 36d and 36e in region 22, the magnitude of the distance "d" between first substrate 26 and wafer 30 may be concurrently determined at differing sites. The distance information for each of beads 36d and 36e is determined as discussed above. Assuming beads 36d and 36e having substantially identical areas, changes in the areas due to first substrate 26 coming into mechanical contact therewith should be substantially the same, were first substrate 26 and wafer 30 substantially parallel and the distance, "d", would be uniform over region 22. Any difference between the areas of beads 36d and 36e after mechanical contact with first substrate 36 may be attributable to first substrate 26 and wafer 30 not being parallel, which could result in a non-uniform distance, "d", between first substrate 26 and wafer in over region 22. Further, the angle θ, formed between first substrate 26 and wafer 30 may be determined from this information, as discussed above. Assuming that areas of beads 36d and 36e differed initially, similar information may be obtained by comparing the relative changes in the areas of beads 36d and 36e that result from mechanical contact with first substrate 26.

0036Specifically, it may be determined whether the distance "d" by analyzing the relative changes between areas of beads 36d and 36e to determine whether first substrate 26 and wafer 30 at regions located proximate to beads 36d and 36e are spaced apart an equal distance "d". If this is the case, then it may be concluded that first substrate 26 and wafer 30 extend parallel to one another. Otherwise, were first substrate 26 and wafer 30 found not to extend parallel to one another, the magnitude of the angle Θ formed therebetween may be determined.

0037Referring to <figref idref="f0001">Figs. 1</figref>, <figref idref="f0002">2</figref> and <figref idref="f0005">10</figref>, another advantage of examining multiple beads in a regions, such as beads 36f, 36g, 36h, 36i. and 36j, is that a shape of either first substrate 26 or wafer 30 may be obtained. This is shown by examining the changes in beads. For example, after compression of beads 36f, 36g, 36h, 36i and 36j by first substrate 26 each is provided with area. 136f, 136g, 136h, 136i and 136j, respectively that defines a compression pattern 137. As shown, beads 136f, and 136j have the greatest area, beads 136g, 136i, have the second greatest area and bead 136h has the smallest area. This may be an indication that first substrate 26 has a concave surface, i.e., is bowed, or that wafer 30 is bowed. From experimental analysis several different information concerning differing types of compression patterns may be obtained to classify and characterize differing shapes or defects in system 10. These may also be employed in look-up table 29 so that processor 25 may match a compression pattern sensed by CCD sensor 23 with a compression pattern in look-up table 29 an automatically ascertain the nature of processing performed by system 10, i.e., whether system is function properly and or acceptable imprints are being generated.

0038CCD sensor 23 may also be implemented for endpoint detection of the spreading of imprinting layer 34 over wafer 30. To that end, one or more pixel of CCD sensor 23 may be arranged to sense a portion of wafer 30. The portion, shown as 87a, 87b, 88a and 88b, in <figref idref="f0004">Fig. 8</figref>, is located in region 22 and is proximate to a periphery of imprinting layer 34 after "d" has reached a desired magnitude. In this fashion, pixels of CCD sensor 23 may be employed as an endpoint detection system that indicates when a desired distance "d" has been achieved, thereby resulting in spreading of beads 36 to form imprinting layer 34 of desired thicknesses. This facilitates determining the magnitude of movement imprint head 12 should undertake in order to facilitate an imprint of imprinting layer 34. To that end, once CCD sensor 23 detects the presence of imprinting layer 34 proximate to portions 87a, 87b, 88a and 88b, data concerning the same is communicated to processor 25. In response, processor 25 operates to halt movement of imprint head 12, fixing the distance, "d" between first substrate 26 and wafer 30. Although:

0039Referring to <figref idref="f0002">Figs. 2</figref>, <figref idref="f0004">7</figref> and <figref idref="f0005">11</figref> in accordance with another embodiment of the present invention, detection system may include one or more photodiodes, four of which are shown as 90a, 90b, 90c and 90d may be included to facilitate endpoint detection. Photodiodes 90a, 90b, 90c and 90d include wave shaping optics 91 and are arranged to sense a predetermined portion of first substrate 26, such as 88a. However, it is advantages to have photodiodes sense portions 88b, 87a and 87b, as well. For ease of discussion however, photodiodes are discussed with respect to region 88a, with the understanding that the present discussion applies equally to use of additional photodiodes to sense regions 87a, 87b and 88b.

0040To facilitate endpoint detections, photodiodes 90a, 90b, 90c and 90d are positioned to a portion of first substrate 26 that is located proximate to a periphery of imprinting layer 34 after "d" has reached a desired magnitude. As a result, photodiodes 90a, 90b, 90c and 90d may be employed as an endpoint detection system as discussed above with respect to CCD sensor 23 shown in <figref idref="f0001">Fig. 1</figref>. Referring again to <figref idref="f0002">Figs. 2</figref> and <figref idref="f0005">11</figref>, photodiodes 90a, 90b, 90c and 90d are in data communication with processor 25 to transmit information concerning portion 88, such as intensity of light reflected from portion 88. Specifically, portion 88 may be reflective, i.e., a mirror reflects ambient onto photodiodes 90a, 90b, 90c and 90d. Upon being covered by imprinting layer 34, the energy of light reflecting from portion 88 is substantially reduced, if not completely attenuated, thereby reducing the power of optical energy impinging upon photodiodes 90a, 90b, 90c and 90d. Photodiodes 90a; 90b, 90c and 90d produce a signal in response thereto that is interpreted by processor 25. In response, processor 25 operates to halt movement of imprint head 12, fixing the distance, "d" between first substrate 26 and wafer 30. It should be understood that the detection system discussed with respect to photodiodes 90a, 90b, 90c and 90d may be used in conjunction with CCD sensor 23 and wave shaping optics 24, discussed with respect to <figref idref="f0001">Fig. 1</figref>. The advantage of employing photodiodes 90a, 90b, 90c and 90d is that data acquisition is faster than that provided by pixels of CCD sensor 23.

0041Referring to <figref idref="f0002">Figs. 2</figref>, <figref idref="f0005">11</figref> and <figref idref="f0006">12</figref>, another embodiment of the present invention that facilitates determining characteristics of first substrate 26 and wafer 30 without knowing the volume associated with beads 36. To that end, the present embodiment of system 110 includes an interferometer 98 that may be used with the CCD sensor 23 the photodiodes 90a, 90b, 90c and 90d or a combination of both. As discussed above, system 110 includes wave shaping optics 24 and radiation source 16 and a mirror 20, imprint head 12. Imprint head 12 retains, first substrate 26 disposed opposite wafer 30, with wafer 30 being support by stage 14. Processor 25 is in data communication with imprint head 12, stage 14, radiation source 16, CCD sensor 23 and interferometer 98. Also disposed in an optical path of interferometer 98 is a 50-50 mirror 25 that enables a beam produced by interferometer to be reflected onto region, while allowing CCD sensor 23 to sense region 22.

0042Use of interferometry facilitates determining distance "d" without having accurate information concerning the initial volume of beads 36. An exemplary interferometry system employed to measure distance, "d", is described in United States patent application number <patcit id="pcit0007" dnum="US10210894B"><text>10/210,894</text></patcit>, entitled "Alignment Systems for Imprint Lithography" .

0043Employing interferometer 98 facilitates concurrently determining the initial distance "d" and the change in distance Δd. From this information the volume associated with one or more beads 36 may be obtained. For example, interferometer 98 may be employed to obtain two measurements of first substrate 26 at two differing times t₁ and t₂ to obtain first substrate displacement measurement L_T. During the same time, wafer 30 displacement measurement, L_s, may be obtained, in a similar manner. The change in distance, Δd, between first substrate 26 and wafer 30 is obtained as follows: <maths id="math0004" num="(4)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>d</mi><mo>=</mo><mfenced open="|" close="|"><msub><mi>L</mi><mi>T</mi></msub><mo>-</mo><msub><mi>L</mi><mi>S</mi></msub></mfenced></math><img file="EP1570249B1_D0005.tif" /></maths> During times t₁ and t₂, measurements are taken with CCD sensor 23 to determine the change in area of one ore more of beads 36 as a function of the total number of pixels in which one or more of beads 36 are sensed. At time t₁, the total number of pixels in which one or more beads 36 are sensed is np₁. At time t₂, the total number of pixels in which one or more beads 36 are sensed is np₂. From these two values the change in pixels, Δn<i>_p</i>, is defined as follows: <maths id="math0005" num="(5)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>n</mi><mi>p</mi></msub><mo>=</mo><mfenced open="|" close="|"><msub><mi>n</mi><mrow><mi>p</mi><mo>⁢</mo><mn>2</mn></mrow></msub><mo>-</mo><msub><mi>n</mi><mrow><mi>p</mi><mo>⁢</mo><mn>1</mn></mrow></msub></mfenced></math><img file="EP1570249B1_D0006.tif" /></maths>

0044From equations 4 and 5 the value of distance, d, may be obtained from either of the following equations: <maths id="math0006" num="(6)"><math display="block"><msub><mi>d</mi><mn>1</mn></msub><mo>=</mo><mfenced><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>d</mi><mo>/</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>n</mi><mi>p</mi></msub></mfenced><mo>⁢</mo><msub><mi>n</mi><mrow><mi>p</mi><mo>⁢</mo><mn>1</mn></mrow></msub></math><img file="EP1570249B1_D0007.tif" /></maths><maths id="math0007" num="(7)"><math display="block"><msub><mi>d</mi><mn>2</mn></msub><mo>=</mo><mfenced><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>d</mi><mo>/</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>n</mi><mi>p</mi></msub></mfenced><mo>⁢</mo><msub><mi>n</mi><mrow><mi>p</mi><mo>⁢</mo><mn>2</mn></mrow></msub></math><img file="EP1570249B1_D0008.tif" /></maths> where <i>d</i> = <i>d₁</i> = <i>d₂</i>. Knowing <i>d₁</i> and <i>d₂</i>, by substitution we can obtain the volume V of the one or more beads 36 being sensed by CCD sensor 23 by either of the following equations: <maths id="math0008" num="(8)"><math display="block"><msub><mi>V</mi><mn>1</mn></msub><mo>=</mo><msub><mi>d</mi><mn>1</mn></msub><mfenced><msub><mi>n</mi><mrow><mi>p</mi><mo>⁢</mo><mn>1</mn></mrow></msub><mo>×</mo><mi mathvariant="italic">pixelsize</mi></mfenced></math><img file="EP1570249B1_D0009.tif" /></maths><maths id="math0009" num="(9)"><math display="block"><msub><mi>V</mi><mn>2</mn></msub><mo>=</mo><msub><mi>d</mi><mn>2</mn></msub><mfenced><msub><mi>n</mi><mrow><mi>p</mi><mo>⁢</mo><mn>2</mn></mrow></msub><mo>×</mo><mi mathvariant="italic">pixelsize</mi></mfenced></math><img file="EP1570249B1_D0010.tif" /></maths> where V = V₁ = V₂, and <i>(</i>n_p1 × <i>pixelsize)</i> = <i>(n_p2</i> × <i>pixelsize)</i> = A. When first substrate 26 and wafer 30 may be maintained to be parallel, interferometer 98 may be measure outside of region 22, shown in <figref idref="f0001">Fig. 1</figref>. Otherwise, interferometer measurements should be made proximate to a center of region 22, or expanding beads 36. In this manner, the substrate characteristic information obtained using system 10, shown in <figref idref="f0001">Fig. 1</figref>, may be obtained employing system 110, shown in <figref idref="f0006">Fig. 12</figref>.

0045The embodiments of the present invention described above are exemplary. The scope of the invention is determined by the appended claims