Vibration and Environmental Isolation Chamber for Thin Wafers

Abstract

The measurement cavity described herein includes a cylindrical chamber having a first open end and a second open end, a first cap covering the first open end of the cylindrical chamber and a second cap covering the second open end of the cylindrical chamber, and a wafer holder disposed within and coupled to the cylindrical chamber. The first and second caps seal the cylindrical chamber, and the first cap is rigidly coupled to the second cap. The cylindrical chamber has a mass m, a stiffness k, and a damping constant c that are set to reduce the transmissibility of an applied force F of 60 Hz in the measurement cavity by a factor of 10 or less, and the measurement cavity has a natural frequency greater than 300 Hz.

Description

Priority

[0001] This application claims the benefit of priority under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 62 / 942,947, filed December 3, 2019, the entire contents of which are incorporated herein by reference. [Technical field]

[0002] FIELD OF THE DISCLOSURE This disclosure relates generally to optical metrology, and more particularly to an apparatus for vibration and environmental isolation of thin wafers in a Fizeau interferometer. [Background technology]

[0003] Interferometers can measure the surfaces of various types of optical components with high precision by utilizing the interference fringes that occur between light reflected from a reference surface and light reflected from a test surface. One such device is the Fizeau interferometer, which can be used to measure various optical surfaces, especially spherical and flat surfaces with relatively large diameters.

[0004] In particular, thin and large-diameter wafers are susceptible to external influences such as floor vibrations, air currents, acoustics, temperature, humidity, etc. However, some industries require sub-nanometer resolution for wafer measurement. Summary of the Invention [Problem to be solved by the invention]

[0005] Therefore, there is a need for an interferometer that isolates the thin wafer from vibrational and environmental forces. [Means for solving the problem]

[0006] A measurement cavity according to a first embodiment disclosed herein includes a cylindrical chamber having a first open end and a second open end, a first cap covering the first open end of the cylindrical chamber and a second cap covering the second open end of the cylindrical chamber, and a wafer holder disposed within and coupled to the cylindrical chamber. The first cap and the second cap seal the cylindrical chamber, and the first cap is rigidly coupled to the second cap. The wafer holder has a first transmissibility |x / F| to an applied force F of 60 Hz. The cylindrical chamber has a mass m, a stiffness k, and a damping constant c that are set such that a second transmissibility |x / F| to an applied force F of 60 Hz in the measurement cavity is at most one tenth of the first transmissibility |x / F|, and the measurement cavity has a natural frequency of more than 300 Hz.

[0007] A measurement cavity according to a second aspect disclosed herein includes the measurement cavity according to the first aspect, wherein the first cap, the second cap, or both the first cap and the second cap are provided with reference optics capable of performing one or more interferometric or optical measurements.

[0008] A measurement cavity according to a third aspect disclosed herein includes the measurement cavity according to the first or second aspect, wherein the first cap and the second cap comprise reference optics capable of performing one or more interferometric or optical measurements.

[0009] A measurement cavity according to a fourth aspect disclosed herein includes the measurement cavity according to any of the first to third aspects, in which the reference optics comprises a glass substrate mounted within a bezel.

[0010] A measurement cavity according to a fifth aspect disclosed herein includes the measurement cavity according to the fourth aspect, in which the bezel is formed of metal.

[0011] A measurement cavity according to a sixth aspect disclosed herein includes the measurement cavity according to the fourth or fifth aspect, wherein the bezel is rigidly coupled to the cylindrical chamber.

[0012] A measurement cavity according to a seventh aspect disclosed herein includes the measurement cavity according to the sixth aspect, wherein the bezel is bolted to the cylindrical chamber.

[0013] A measurement cavity according to an eighth aspect disclosed in this specification includes the measurement cavity according to any of the fourth to seventh aspects, in which the glass substrate is sealed within the bezel by epoxy.

[0014] A measurement cavity according to a ninth aspect disclosed herein includes the measurement cavity according to any of the above aspects, wherein the wafer holder is rigidly coupled to the cylindrical chamber.

[0015] A measurement cavity according to a tenth aspect disclosed in this specification includes the measurement cavity according to any of the first to eighth aspects, in which the wafer holder is coupled to the cylindrical chamber via one or more springs.

[0016] A measurement cavity according to an eleventh aspect disclosed in the present specification includes the measurement cavity according to any of the above aspects, wherein the wafer holder is configured to hold the wafer without contacting the first and second sides of the wafer.

[0017] A measurement cavity according to a twelfth aspect disclosed herein includes the measurement cavity according to any of the above aspects, wherein the first cap is rigidly connected to the second cap by clamping.

[0018] A measurement cavity according to a thirteenth aspect disclosed in this specification includes a measurement cavity according to any of the first to eleventh aspects, in which the first cap is rigidly connected to the second cap via one or more bolts.

[0019] A measurement cavity according to a fourteenth aspect disclosed herein includes the measurement cavity according to any of the above aspects, wherein the cylindrical chamber provides a gap between the first surface of the first cap and the first surface of the second cap.

[0020] A measurement cavity according to a fifteenth aspect disclosed herein comprises the measurement cavity according to any of the previous aspects, wherein the cylindrical chamber is formed from a metal or metal-based substrate.

[0021] A measurement cavity according to a sixteenth aspect disclosed herein includes the measurement cavity according to any of the above aspects, wherein the cylindrical chamber is formed from an aluminum-based substrate or stainless steel.

[0022] A measurement cavity according to a seventeenth aspect disclosed herein includes the measurement cavity according to any of the above aspects, wherein the first cap and the second cap are attached in a configuration that does not deform one another.

[0023] According to an eighteenth aspect disclosed herein, an interferometer includes a measurement cavity according to any of the above aspects.

[0024] According to a nineteenth aspect, the interferometer includes the interferometer according to the eighteenth aspect, performing measurements from either the first cap or the second cap.

[0025] According to a twentieth aspect, the interferometer includes the interferometer according to the eighteenth aspect, which is a dual interferometer performing measurements from either the first cap or the second cap.

[0026] Additional features and advantages are set forth in the detailed description that follows, and will become apparent to those skilled in the art in part from the description, or may be learned by practice of the embodiments described herein, including the following detailed description, claims, and accompanying drawings.

[0027] It should be understood that both the foregoing general description and the following detailed description are intended to describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. In addition, the accompanying drawings are included to provide a further understanding of the various embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate by way of example the various embodiments described herein and, together with the following detailed description, serve to explain the principles and operation of the claimed subject matter. [Brief description of the drawings]

[0028] [Figure 1A] FIG. 1 is a diagram of a dual interferometer including a measurement cavity according to one or more embodiments shown and described herein. [Figure 1B] FIG. 2 is a diagram of a measurement cavity according to one or more embodiments shown and described herein, illustrating multiple reflections that produce interference fringes within the measurement cavity. [Diagram 2] 1 is a cross-sectional view of a measurement cavity with a wafer therein according to one or more embodiments shown and described herein; [Diagram 3] FIG. 1 illustrates a top view of an optical reference, which may take the form of a cap for a measurement cavity according to one or more embodiments shown and described herein. [Figure 4] FIG. 2 is a cross-sectional view of a measurement cavity with a wafer placed therein and held by a spring-loaded wafer holder in accordance with one or more embodiments shown and described herein. [Diagram 5] FIG. 2 is a top view of a measurement cavity with the first cap removed to reveal the wafer holder according to one or more embodiments shown and described herein. [Figure 6] 1 is a Bode plot of transmissibility (Y-axis) as a function of frequency (X-axis in Hertz (Hz)) for varying the mass of a measurement cavity according to one or more embodiments shown and described herein; [Figure 7]1 is a Bode plot of transmissibility (Y-axis) as a function of frequency (X-axis in Hertz (Hz)) for varying stiffness of a measurement cavity according to one or more embodiments shown and described herein. [Figure 8] 1 is a Bode plot of transmissibility (Y-axis) as a function of frequency (X-axis in Hertz (Hz)) for varying damping of a measurement cavity according to one or more embodiments shown and described herein; [Figure 9] 1 is a Bode plot of transmissibility (Y-axis) as a function of frequency (X-axis, in Hertz (Hz)) for the wafer mount alone and for the wafer mount placed in a measurement cavity according to one or more embodiments shown and described herein; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Reference will now be made in detail to various embodiments, as illustrated in the accompanying drawings, and wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

[0030] In the present specification, ranges may be expressed in the form of "about" a certain value or more, "about" a certain value to "about" another certain value, or "about" the other certain value or less. When a range is expressed in this manner, there are other embodiments including the certain value or more, the certain value to the other certain value, or the other certain value or less. Similarly, when a value is expressed as an approximation by using "about", it will be understood that there are other embodiments consisting of the specific value itself. It will also be understood that the endpoints of each range have meanings relative to each other as well as independent of each other.

[0031] As used herein, directional terms (e.g., "up," "down," "right," "left," "front," "back," "top," "bottom," etc.) are used with reference to the drawings only and are not intended to imply absolute orientations.

[0032] Unless otherwise expressly stated, no method described herein is intended to be construed as requiring that steps be performed in a particular order, nor is any method described herein intended to require a particular orientation of any apparatus. Thus, unless a method claim actually recites an order of its steps, or unless the claims or detailed description are otherwise expressly stated to be limited to a particular order, or unless a particular sequence or orientation of components of an apparatus is recited, no sequence or orientation is intended to be inferred in any respect. This applies to all non-express matters that may provide a basis for interpretation, such as matters of reasoning regarding sequence of steps, operational flow, sequence of components, or orientation of components, common meaning derived from grammatical construction or punctuation, the number or type of embodiments described herein, etc.

[0033] As used herein, the singular forms "a," "an," and "the" are intended to include reference to the corresponding plurals unless the context clearly indicates otherwise. Thus, for example, an expression introducing a certain component with the article "a" also includes embodiments having two or more of that component, unless the context clearly indicates otherwise.

[0034] Additionally, the term "formed from" can mean one or more of "comprises," "consists essentially of," or "consists of." For example, when a component is "formed of" a particular material, the component may comprise, consist essentially of, or consist of the particular material.

[0035] FIG. 1A illustrates an interferometer 10 according to various embodiments. The interferometer 10 illustrated in FIG. 1A is a dual interferometer including an upper interferometer 12 and a lower interferometer 14 for measuring first and second opposing sides 16 and 18 of a wafer 106. It is noted that while FIG. 1A illustrates the interferometer 10 as a dual interferometer configured to measure the wafer 106 from the first and second sides 16 and 18, other types of interferometers are contemplated, including a single interferometer that measures the wafer 106 from one side. Additionally, although the interferometer 10 is described herein as being used to measure the wafer 106, it is contemplated that the interferometer 10 can be used to measure any opaque test part that is made of a material that does not transmit frequencies in the band propagating through the interferometers 12 and 14, or that has sufficient scattering properties that the frequencies in the band cannot be transmitted with any regularity.

[0036] The upper interferometer 12 includes a first illuminator 22. The first illuminator 22 may include a conventional light source 26 for outputting a coherent first measurement beam 46, and a beam shaper 30. The lower interferometer 14 includes a second illuminator 24. The second illuminator 24 may include a conventional light source 28 for outputting a coherent second measurement beam 48, and a beam shaper 32. For example, the light sources 26, 28 may be semiconductor diode lasers, and the beam shapers 30, 32 may include beam expanders and beam conditioners for changing the light distribution of the measurement beams 46, 48.

[0037] In the upper interferometer 12, a first measurement beam 46 propagates through a first shutter 34 to a first beam splitter 38 which directs (e.g., transmits) the first measurement beam 46 into a first measurement arm 42. In the lower interferometer 14, a second measurement beam 48 propagates through a second shutter 36 to a second beam splitter 40 which directs (e.g., transmits) the second measurement beam 48 into a second measurement arm 44. To prevent light from one interferometer 12, 14 from mixing with light from the other interferometer 14, 12, the opening and closing of the first shutter 34 and the second shutter 36 can be coordinated by a common processor / controller 100 to alternately block the propagation of either the first measurement beam 46 or the second measurement beam 48. The first beamsplitter 38 and the second beamsplitter 40 may take the form of pellicle beamsplitters, beamsplitter cubes, or beamsplitter plates of the amplitude splitting or polarizing type.

[0038] Measurement arm 42 includes dual function optics 50 within housing 51. Measurement arm 44 includes dual function optics 52 within housing 53. Dual function optics 50, 52 contribute to both illuminating and imaging wafer 106. The illumination function of dual function optics 50, 52 allows for sizing and shaping of the wavefronts of measurement beams 46, 48 to nominally approximately match the shape of opposing surfaces 16, 18 of wafer 106.

[0039] As shown in FIG. 1B, the first measurement arm 42 further comprises a first cap 206, which includes a reference optic (e.g., a Fizeau wedge) having a first reference surface 58 for reflecting a portion of the first measurement beam 46 as a reference beam 62. The second measurement arm 44 further comprises a second cap 208, which includes a reference optic (e.g., a Fizeau wedge) having a second reference surface 60 for reflecting a portion of the second measurement beam 48 as a reference beam 64. The reference optic is selected to transmit frequencies in the band propagating through the interferometers 12, 14. It is noted that while various embodiments herein are described as including the reference optics in the first cap 206 and the second cap 208, it is contemplated that in some embodiments the reference optics may be separate from the caps. Meanwhile, the remaining portions of the measurement beams 46, 48 propagate through the first cap 206 and the second cap 208, and further portions of the remaining portions of the measurement beams 46, 48 traverse the first cap 206 and the second cap 208 and are reflected from the opposing faces 16, 18 of the wafer 106 as object beams 66, 68.

[0040] The object beam 66 combines with the reference beam 62 at the reference surface 58 to form interference fringes (not shown) that represent the difference between the first surface 16 and the reference surface 58. The object beam 68 also combines with the reference beam 64 at the reference surface 60 to form interference fringes (not shown) that represent the difference between the second surface 18 and the reference surface 60.

[0041] The reflected object beam 66 and reference beam 62 both propagate along a common optical path in measurement arm 42 to beam splitter 38, where at least a portion of beam 66 and beam 62 are directed (e.g., reflected) into a recording arm 78 of upper interferometer 12. Similarly, the reflected object beam 68 and reference beam 64 both propagate along a common optical path in measurement arm 44 to beam splitter 40, where at least a portion of beam 68 and beam 64 are directed (e.g., reflected) into a recording arm 80 of lower interferometer 14.

[0042] 1A, in the recording arm 78, the interference fringes formed on the first reference surface 58 are imaged onto the detection surface 82 of the camera 86. Similarly, in the recording arm 80, the interference fringes formed on the second reference surface 60 are imaged onto the detection surface 84 of the camera 88. The detection surfaces 82, 84 may include detector arrays for measuring the beam intensity across a field of view encompassing the opposing surfaces 16, 18 of the wafer 106. The dual function optics 50, 52 may contribute to forming the reference image on the detection surface 82, 84. However, the cameras 86, 88 may include or be coupled to the imaging optics 74, 76 for resizing or otherwise polishing the imaging of the reference image onto the detection surface 82, 84.

[0043] 1A and 1B, a first cap 206 and a second cap 208 are physically interconnected by a cylindrical chamber 204 to form a measurement cavity 200. The measurement cavity 200 protects and preserves the overall integrity of the optical reference cavity 108 defined by the two reference surfaces 58, 60. In addition, in various embodiments, the measurement cavity isolates the wafer 106 from environmental and vibrational forces. An example of a measurement cavity 200 is shown in FIG. 2. Specifically, FIG. 2 illustrates a cross-sectional view of the measurement cavity 200 with the wafer 106 placed therein.

[0044] 2, the wafer 106 is secured within the measurement cavity 200 by a wafer holder 202 that is disposed within and coupled to a cylindrical chamber 204. The cylindrical chamber 204 has a first open end and a second open end opposite the first open end. A first cap 206 covers the first open end of the cylindrical chamber 204, and a second cap 208 covers the second open end of the cylindrical chamber 204. In various embodiments, the first cap 206 and the second cap 208 seal the cylindrical chamber 204.

[0045] In various embodiments, the cylindrical chamber 204 is formed of a metal or metal-based substrate. For example, the cylindrical chamber 204 may be formed of an aluminum-based substrate or stainless steel. Other materials are contemplated as long as they are capable of insulating the interior of the measurement cavity from the outside. It is noted that while in the embodiment shown in FIG. 2 the cylindrical chamber 204 includes a lip that supports the wafer holder 202, in some embodiments the interior of the cylindrical chamber 204 may be defined by a straight, flat wall from the first open end to the second open end. Other configurations are possible and contemplated and may be selected based on the particular application of the cylindrical chamber 204.

[0046] In various embodiments, the first cap 206 is rigidly coupled to the second cap 208. Rigidly coupling the first cap 206 and the second cap 208 maintains the proper positional relationship between the first cap 206 and the second cap 208, allowing them to be used as Fizeau optics and reference surfaces, while also reducing errors and improving resolution, as described in more detail below. In some embodiments, the cylindrical chamber 204 provides a gap between the reference surface 58 of the first cap 206 and the reference surface 60 of the second cap 208. For example, as shown in FIG. 2, the first cap 206 and the second cap 208 can be in contact with corresponding ends of the cylindrical chamber 204, respectively, to maintain a constant distance between the reference surface 58 and the reference surface 60.

[0047] In some embodiments, a mechanical structure other than the cylindrical chamber 204 is used to provide a gap between the reference surface 58 and the reference surface 60. For example, two or more spacer pins (not shown) may be used to define the gap between the reference surface 58 and the reference surface 60. The mechanical structure, such as the spacer pins, may be made of a thermally stable material such as Invar. Each spacer pin may penetrate a recess in the cylindrical chamber 204 and contact the first cap 206 and the second cap 208. From a kinematic perspective, the rigid coupling between the first cap 206 and the second cap 208 ensures that the reference surface 58 and the reference surface 60 move in the same way as if they were part of a single object. That is, any movement that affects one of the two reference surfaces 58, 60 will have the same effect on the other of the two reference surfaces 60, 58. In various embodiments, the gap between reference surface 58 and reference surface 60 is substantially uniform (eg, the gap varies less than 10 μm across the reference surfaces).

[0048] In various embodiments, the first cap 206 and the second cap 208 are configured to have low or no stress between the caps so that any rigid connection between the first cap 206 and the second cap 208 will not cause distortion of the Fizeau optics. In various embodiments, the relative configuration of the first cap 206 and the second cap 208 is such that the deformation of the first cap 206 and the second cap 208 due to this is limited to less than 10 μm. In some embodiments, the first cap 206 and the second cap 208 are attached in a configuration that does not deform each other.

[0049] As described above, in various embodiments, the first cap 206, the second cap 208, or both the first cap 206 and the second cap 208 include a reference optic that can perform one or more interferometric or optical measurements. If the reference optic is a Fizeau optic, the first cap 206 and the second cap 208 can be wedge-shaped with one side angled at about 1 degree relative to the opposite side. This angle is set so that reflections from the inclined surface do not return to the camera and interfere with the specimen beam or the reference beam. It should be understood that other reference optic designs are possible and optical measurements can be performed from one or both sides of the wafer 106.

[0050] In various embodiments, at least a portion of the cap can be formed of an optically transparent material, such as glass. In some embodiments, such as the embodiment shown in FIG. 2, the first cap 206 and / or the second cap 208 can be entirely glass substrates, and the entire cap can be the optical reference. In some embodiments, as shown in FIG. 3, the optical reference 300 can include a Fizeau wedge 302 mounted within an annular housing, such as a bezel 304. An adhesive, such as an epoxy system, can be used to secure the Fizeau wedge 302 within the bezel 304.

[0051] The bezel 304 may be made of a metal such as aluminum or stainless steel, or other material sufficient to rigidly hold the Fizeau wedge 302 in place and form a tight seal with the cylindrical chamber 204. In various embodiments, the bezel 304 may be rigidly coupled to the cylindrical chamber 204 and secured in place with adhesive or bolts. For example, a bolt 402 (shown in FIG. 4) may pass through a hole 306 in the bezel 304 to secure the optical reference 300 to the cylindrical chamber 204 (not shown in FIG. 3).

[0052] In various embodiments, such as those in which the first cap 206 and the second cap 208 are formed entirely of a glass substrate and do not have a bezel 304 and therefore cannot be bolted to the cylindrical chamber 204, an adhesive, such as an epoxy system, may be used to bond the first cap 206 and / or the second cap 208 to the cylindrical chamber 204. However, in various embodiments, the first cap 206 and / or the second cap 208 may be removably sealed to the cylindrical chamber 204 in some embodiments such that one or both of the first cap 206 and the second cap 208 may be removed from the cylindrical chamber 204, such as during insertion or removal of the wafer 106. For example, the first cap 206 and the second cap 208 may be positioned over a first open end of the cylindrical chamber 204, and the second cap 208 may be positioned over a second open end of the cylindrical chamber 204, and may be clamped or otherwise secured in place.

[0053] 2, in some embodiments, a gasket 210, such as an elastomeric seal, such as an O-ring, or a metallic seal, such as a braze, is disposed between each cap and the cylindrical chamber 204 to ensure that the measurement cavity 200 is airtight. Without being bound by theory, making the chamber airtight can reduce or eliminate the effects of air currents on the wafer 106, as well as provide acoustic isolation for the wafer 106. Additionally, making the measurement cavity 200 airtight can also reduce the effects of temperature and humidity fluctuations on the wafer 106.

[0054] As shown in Figures 2, 4, and 5, the wafer 106 is positioned by a wafer holder 202. The wafer holder 202 may be rigidly coupled to the cylindrical chamber 204 or may be coupled to the cylindrical chamber 204 via a damping material. For example, the wafer holder 202 may be bolted or glued to the cylindrical chamber 204. Alternatively, as shown in Figures 4 and 5, the wafer holder 202 may be coupled to the cylindrical chamber 204 via one or more springs 404.

[0055] 4 and 5, the wafer holder 202 is coupled to the cylindrical chamber 204 via one or more actuators 406 and springs 404. The actuators 406 and springs 404 allow the wafer holder 202 to be raised, lowered, or tilted relative to the cylindrical chamber 204 depending on the particular embodiment. The springs 404 provide a restoring force that keeps the actuator in contact with a V-shaped portion 408 attached to the second cap 208. In various embodiments, the stiffness of the springs can be varied to vary the damping or transmissibility of the force transmitted from the cylindrical chamber 204 to the wafer holder 202 and thus to the wafer 106.

[0056] The wafer holder 202 further comprises one or more wafer mounts 410 configured to receive the wafer 106. In the embodiment shown in FIG. 5, the wafer mounts 410 form a three-point support that secures the wafer 106 within the measurement cavity 200. However, it is contemplated that the wafer holder 202 and / or the wafer mounts 410 may have other configurations. For example, in some embodiments, the wafer holder 202 may be configured such that it does not contact the first side 16 and the second side 18 of the wafer 106, or the wafer holder 202 may simply be configured to prevent the wafer 106 from moving beyond a defined range, such as in embodiments that use air bearings to position the wafer 106. Also, in some embodiments, the wafer holder 202 is configured to obscure as little of the wafer 106 as possible. Regardless of the particular configuration employed for the wafer mount 410 and wafer holder 202, in various embodiments, the wafer 106 is disposed within the measurement cavity 200 in a “free state” or a “quasi-free state.” In other words, the first side 16 and second side 18 of the wafer 106 are not attached to, in contact with, or pressed against another structure, such as the wafer holder 202.

[0057] 1A again, in order to keep the distance and the tilt between the opposing faces 16, 18 of the wafer 106 and the reference faces 58, 60 of the caps 206, 208 constant while illuminating the wafer 106 with a portion of the measurement beams 46, 48 to perform measurements, the wafer 106 is mounted in the measurement cavity 200 as if it were a part of the measurement cavity 200. This minimizes the influence of disturbances acting on the entire measurement cavity 200 on the interference fringes formed in the upper interferometer 12 and the lower interferometer 14. Furthermore, at least one cap (e.g., the first cap 206) is separated from the other part of the corresponding measurement arm (e.g., the measurement arm 42 in FIG. 1A). For example, the housing 51 of the measurement arm 42 does not have a direct physical connection with the first cap 206 and is independent of the first cap 206 mounted as a part of the measurement cavity 200. Instead, the housing 51 of the measurement arm 42 is attached to the base 124 via a flange 120 and a collar 122. The base 124 preferably has a large mass (e.g. a granite slab or a steel plate) to isolate the upper interferometer 12 from environmental disturbances. However, the measurement cavity 200 is connected to the housing 53 of the measurement arm 44 via a collar 126, and the housing 53 of the measurement arm 44 is connected to the base 124 via a flange 128. By isolating one cap 206 (and thus one optical reference) from the rest of its measurement arm 42, the two caps 206, 208 are insulated from disturbances such as movements or movements not intended for the other of the measurement arms 42, 44. Isolating the measurement cavity 200 from one of the measurement arms 42 reduces the structural requirements that the measurement cavity 200 must meet to maintain a constant spacing and relative tilt between the two reference surfaces 58, 60. It is contemplated that in some embodiments the measurement cavity 200 may be decoupled from both of the measurement arms 42, 44.

[0058] Without being bound by theory, it is believed that resonant frequencies in the environment may induce non-contact vibrations in the optical elements (including the first cap 206 and the second cap 208), the wafer 106, or both. Therefore, in various embodiments, the measurement cavity 200 is designed to reduce or minimize vibration transmission. Specifically, to achieve measurement of the wafer 106 with sub-nanometer precision, in various embodiments, the measurement cavity 200 is specifically designed to reduce or block a band of frequencies that may be transmitted based on the natural vibration frequency of the wafer 106.

[0059] Vibrations from the external environment are transferred as time-dependent forces (e.g., oscillating forces) to the interferometer 10 and through the interferometer 10 to the measurement cavity 200. Such forces may move the wafer 106 relative to the first cap 206 and / or the second cap 208, thereby adversely affecting the resolution of measurements made on the first and second sides 16 and 18 of the wafer 106. Such movement of the wafer 106 may also affect the alignment and spacing between the first side 16 and the reference surface 58 and between the second side 18 and the reference surface 60. It is therefore desirable to minimize the transmission of vibrational forces into and through the measurement cavity 200.

[0060] The force transmission ratio |x / F| in a system as a function of mass, stiffness, and damping is given by:

number

[0061] In various embodiments, in addition to the techniques described above, the measurement cavity 200 has a large mass to reduce the effects of disturbances on the optical components (e.g., caps 206, 208, Fizeau wedge 302, etc.) and / or wafer 106. The mass m of the measurement cavity 200 can change the natural frequency of the measurement cavity 200, increasing the amount of energy (force) required to excite (oscillate) the measurement cavity 200.

[0062] FIG. 6 is a Bode plot of equation (1) showing the effect of the mass of the measurement cavity on the transmissibility of an applied force of magnitude F and displacement x as a function of the frequency w of the applied force. Of particular interest here is the transmissibility of an applied force of 60 Hz, which corresponds to the natural frequency of a 300 mm wafer and is also the standard power frequency in the United States. In other words, the power in the building may generate a vibratory applied force sufficient to induce vibrations in the 300 mm wafer in the measurement cavity 200. Thus, suppression of the applied force of 60 Hz is an important design goal for improving the interferometric resolution of the measurement cavity.

[0063] In Figure 6, the mass of the measurement cavity was changed to calculate the transmissibility. As shown in Figure 6, when the mass was increased from 1 to 8 times, the transmissibility of the applied force of 60 Hz increased by approximately 1.00E-05 (1.00 × 10 -5 ) to approximately 1.00E-06(1.00×10 -6 ) as the natural frequency of the system (corresponding to the frequency of the sharp peak in FIG. 6, which represents the frequency of the force most efficiently transmitted within the measurement cavity) is also found to decrease. Thus, in various embodiments, the measurement cavity has a mass effective to reduce the transmissibility of an applied force of 60 Hz by a factor of at least 10. In various embodiments, the mass of the measurement cavity is selected such that the natural frequency of the measurement cavity is greater than 300 Hz and the mass of the measurement cavity is greater than 10 times the mass of the wafer holder 202 alone.

[0064] As shown in equation (1) above, the stiffness of the system also affects the transmissibility of the applied force. FIG. 7 is a Bode plot showing the effect of the stiffness of the measurement cavity on the transmissibility of an applied force of magnitude F and displacement x as a function of the frequency w of the applied force according to calculations based on equation (1). In FIG. 7, the stiffness of the measurement cavity was varied to calculate the transmissibility. As shown in FIG. 7, increasing the stiffness reduces the transmissibility of an applied force of frequency 60 Hz, similar to the case of increasing the mass. However, increasing the stiffness did not change the natural frequency of the system. Thus, in various embodiments, the measurement cavity 200 has a stiffness effective to reduce the transmissibility of an applied force of 60 Hz by at least a factor of 10. In various embodiments, the stiffness of the measurement cavity is selected such that the natural frequency of the measurement cavity is greater than 300 Hz.

[0065] The damping of the system also affects the transmissibility of the applied force. Figure 8 is a Bode plot showing the effect of the damping constant c of the measurement cavity on the transmissibility of an applied force of magnitude F and displacement x as a function of the frequency w of the applied force, according to a calculation based on equation (1). In Figure 8, the transmissibility was calculated by varying the damping constant c of the measurement cavity. As shown in Figure 8, increasing the damping constant c has no effect on the transmissibility at frequencies other than the natural frequency, and has no effect at 60 Hz. EXAMPLES

[0066] To demonstrate the functional performance of the measurement cavity, a representative measurement cavity 200 was compared with its associated wafer holder 202. The measurement cavity 200 has a mass of 50 kg, a first natural frequency of approximately 450 Hz, and a stiffness of 1×10 7 The wafer holder (mount) 202 alone, which does not use such a cavity, has a mass of 1 kg, a primary resonance frequency of 300 Hz, and a rigidity of 4×10 4N / m. Figure 9 shows the Bode plot obtained using these numerical examples and equation (1). Figure 9 compares the relative transmissibility of wafer holder 202 and measurement cavity 200. At the problematic frequency of 60 Hz, placing wafer holder (mount) 202 inside measurement cavity 200 reduced the transmissibility by four orders of magnitude (to 1 / 10,000). The natural frequency of the system also reduced by a factor of 7. Note that in the comparison, the damping factor was assumed to be constant and set to 0.01 for the calculations.

[0067] Thus, in various embodiments, the mass, stiffness, and / or damping constant of the measurement cavity 200 are selected to reduce the transmission rate of applied forces generated outside the measurement cavity 200 through the measurement cavity 200. Examples of such applied forces include floor vibrations, electrical noise, and acoustic noise. In various embodiments, the measurement cavity 200 is also airtight, thereby reducing the effect of air currents on the wafer 106 and acoustically isolating the wafer 106. The above-mentioned characteristics of the measurement cavity 200 further reduce the effect of temperature and humidity fluctuations on the wafer 106. Thus, various embodiments herein provide a measurement cavity capable of achieving optical measurements with improved resolution.

[0068] In some embodiments, the wafer holder has a first transmissibility |x / F| for an applied force F of 60 Hz, and the cylindrical chamber has a mass m, stiffness k, and damping constant c configured such that the measurement cavity has a second transmissibility |x / F| for an applied force F of 60 Hz, where the second transmissibility |x / F| is at least 3, 5, 10, or 20 times smaller than the first transmissibility |x / F| when the measurement cavity has a natural frequency greater than 300 Hz.

[0069] It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the subject matter described in the claims. Therefore, as long as the modifications and variations of the various embodiments described herein do not depart from the scope of the appended claims and their equivalents, such modifications and variations are intended to be included within the scope of this specification.

[0070] Preferred embodiments of the present invention will be described below in detail.

[0071] EMBODIMENT 1 a cylindrical chamber having a first open end and a second open end; a first cap covering the first open end of the cylindrical chamber and a second cap covering the second open end of the cylindrical chamber, the first cap and the second cap sealing the cylindrical chamber, the first cap being rigidly connected to the second cap; a wafer holder disposed within the cylindrical chamber and coupled to the cylindrical chamber, the wafer holder having a first transmissibility |x / F| for an applied force F of 60 Hz; A measurement cavity comprising: the cylindrical chamber has a mass m, a stiffness k, and a damping constant c that are set such that a second transmissibility |x / F| for an applied force F of 60 Hz in the measurement cavity is less than or equal to one tenth of the first transmissibility |x / F|; A measurement cavity, the measurement cavity having a natural frequency of greater than 300 Hz.

[0072] EMBODIMENT 2 2. The measurement cavity of embodiment 1, wherein the first cap, the second cap, or both the first cap and the second cap are equipped with a reference optical system capable of performing one or more interferometric or optical measurements.

[0073] EMBODIMENT 3 3. The measurement cavity of embodiment 2, wherein the first cap and the second cap comprise reference optics capable of performing one or more interferometric or optical measurements.

[0074] EMBODIMENT 4 4. The measurement cavity of embodiment 2 or 3, wherein the reference optics comprises a glass substrate mounted in a bezel.

[0075] EMBODIMENT 5 5. The measurement cavity of embodiment 4, wherein the bezel is formed of metal.

[0076] EMBODIMENT 6 6. The measurement cavity of embodiment 4 or 5, wherein the bezel is rigidly connected to the cylindrical chamber.

[0077] EMBODIMENT 7 7. The measurement cavity of embodiment 6, wherein the bezel is bolted to the cylindrical chamber.

[0078] EMBODIMENT 8 8. The measurement cavity according to any one of embodiments 4 to 7, wherein the glass substrate is sealed within the bezel by epoxy.

[0079] EMBODIMENT 9 9. The measurement cavity according to any one of embodiments 1 to 8, wherein the wafer holder is rigidly coupled to the cylindrical chamber.

[0080] EMBODIMENT 10 9. The measurement cavity of any one of embodiments 1 to 8, wherein the wafer holder is coupled to the cylindrical chamber via one or more springs.

[0081] EMBODIMENT 11 11. The measurement cavity of any one of embodiments 1 to 10, wherein the wafer holder is configured to hold the wafer without contacting the first and second sides of the wafer.

[0082] EMBODIMENT 12 12. The measurement cavity according to any one of the previous embodiments, wherein the first cap is rigidly connected to the second cap by clamping.

[0083] EMBODIMENT 13 12. The measurement cavity according to any one of embodiments 1-11, wherein the first cap is rigidly connected to the second cap via one or more bolts.

[0084] EMBODIMENT 14 14. The measurement cavity of any one of embodiments 1 to 13, wherein the cylindrical chamber provides a gap between a first surface of the first cap and a first surface of the second cap.

[0085] EMBODIMENT 15 15. The measurement cavity according to any one of embodiments 1-14, wherein the cylindrical chamber is formed of a metal or metal-based substrate.

[0086] EMBODIMENT 16 16. The measurement cavity of embodiment 15, wherein the cylindrical chamber is formed from an aluminum-based substrate or stainless steel.

[0087] EMBODIMENT 17 17. The measurement cavity according to any one of embodiments 1 to 16, wherein the first cap and the second cap are attached in a configuration that does not deform one another.

[0088] EMBODIMENT 18 An interferometer comprising the measurement cavity according to any one of the first to seventeenth embodiments.

[0089] EMBODIMENT 19 19. The interferometer of embodiment 18, wherein measurements are taken from either the first cap or the second cap.

[0090] EMBODIMENT 20 20. The interferometer of embodiment 18 or 19, which is a dual interferometer that performs measurements from either the first cap or the second cap. [Explanation of symbols]

[0091] 10 Interferometer 12 Upper interferometer 14 Lower interferometer 16 First Side 18 The Second Side 22 First Illuminator 24 Second Illuminator 26,28 light source 30,32 Beam shaper 34 First Shutter 36 Second Shutter 38 First beam splitter 40 Second beam splitter 42 First measurement arm 44 Second measurement arm 46 First measurement beam 48 Second measurement beam 50,52 Dual function optics 51,53 Housing 58 First Reference Plane 60 Second Reference Plane 62,64 Reference beam 66,68 Specimen beam 74 Imaging Optical System 78,80 Recording Arm 82,84 Detection surface 86,88 Camera 100 processors / controllers 106 Wafers 108 Optical Reference Cavity 120,128 Flange 122,126 Color 124 Base 200 measurement cavity 202 Wafer holder 204 Cylindrical Chamber 206 First Cap 208 Second Cap 210 Gasket 300 Optical Reference 302 Fizeau Wedge 304 Bezel 306 holes 402 Volts 404 Spring 406 Actuator 408 V-shaped part 410 Wafer Mount

Claims

1. a cylindrical chamber having a first open end and a second open end; a first cap covering the first open end of the cylindrical chamber and a second cap covering the second open end of the cylindrical chamber, the first cap and the second cap sealing the cylindrical chamber, the first cap being rigidly connected to the second cap; a wafer holder disposed within and coupled to the cylindrical chamber, the wafer holder having a first transmissibility |x / F| for an applied force F of 60 Hz; x is a displacement of a wafer along an optical axis extending between the first open end and the second open end; the cylindrical chamber has a mass m, a stiffness k, and a damping constant c set such that a second transmissibility |x / F| for the applied force F at 60 Hz in the measurement cavity is less than or equal to one tenth of the first transmissibility |x / F|; A measurement cavity, the measurement cavity having a natural frequency of greater than 300 Hz.

2. The measurement cavity of claim 1 , wherein the first cap, the second cap, or both the first cap and the second cap comprise reference optics capable of performing one or more interferometric or optical measurements.

3. The measurement cavity of claim 2 , wherein the first cap and the second cap comprise reference optics capable of performing one or more interferometric or optical measurements.

4. 4. The measurement cavity of claim 2 or 3, wherein the reference optics comprises a glass substrate mounted within a bezel.

5. The measurement cavity of any one of claims 1 to 4, wherein the wafer holder is rigidly coupled to the cylindrical chamber.

6. The measurement cavity of any one of claims 1 to 5, wherein the wafer holder is configured to hold the wafer without contacting the first and second sides of the wafer.

7. The measurement cavity of any one of claims 1 to 6, wherein the cylindrical chamber provides a gap between a first surface of the first cap and a first surface of the second cap.

8. The measurement cavity according to any one of claims 1 to 7, wherein the cylindrical chamber is formed from an aluminium based substrate or stainless steel.

9. The measurement cavity of any one of claims 1 to 8, wherein the first cap and the second cap are attached in a mutually non-deforming configuration.

10. An interferometer comprising a measurement cavity according to any one of the preceding claims.

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