Recent advancements in metrology and nanofabrication systems have created a demand for wafer alignment mechanisms with positioning repeatability on the order of microns to tens of microns. Conventional techniques for passive wafer alignment are time consuming, inaccurate or impractical. For example, optical alignment systems are time consuming to use. Alignment systems that rely on MEMS-based structures on the wafer are impractical for most applications. Conventional alignment mechanisms that use pin constraints also have a tendency to jam.
One of the major challenges in nanoscale manufacturing is defect control. Optical inspection is not an option at the nanoscale level due to the diffraction limit of light, and without inspection high scrap rates can occur. One solution to this problem is inline metrology using atomic force microscopes. Single chip MEMS-based atomic force microscopes (AFMs) have been developed that could be incorporated into fabrication lines. However, metrology with these AFM chips requires accurate placement of specimens relative to the AFM tip. Further, present set up times for an AFM are on the order of thirty minutes.
Conventional passive wafer alignment mechanisms typically consist of a stage with three pins and a nesting force in contact with a wafer. Typically, two pins are in contact with the main flat on the wafer. One conventional design references restoring moments when justifying the location of the third pin, but the position of the nesting force (e.g., preload force) is arbitrarily chosen. In addition, conventional passive wafer alignment systems tend to yield poor positioning repeatability.
Disclosed herein are devices and methods for passive alignment of semiconductor wafers. In accordance with the present disclosure, a mechanical wafer alignment device is provided that enables inline atomic force microscope (AFM) metrology in nanoscale manufacturing and reduces AFM metrology setup time. An example water alignment device can include three pins that constrain the wafer in six degrees of freedom and a nesting force applied by a flexure to keep the wafer in contact with the pins.
The devices and methods described herein are provided for rapid and repeatable passive alignment of samples such as silicon wafers. An example method comprises calculating optimal pin locations of three pins to exactly constrain an object in one plane and determining the ideal location of the third pin, as well as an ideal location of the nesting force, with respect to the edge of the object in contact with the two pins with parallel lines of action. An example wafer alignment device comprises three pins and a flexural bearing that applies a nesting force to keep the three pins in contact with the wafer. In order to increase repeatability out-of-plane each pin may have a taper so that the wafer is constrained in all six degrees of freedom. The nesting force can be applied by a flexural bearing designed to minimize jamming between the wafer and two pins.
An example passive wafer alignment device can include a stage for holding a wafer, a plurality of pins arranged on the stage, the pins being arranged to contact respective portions of the wafer, and a preload device arranged on the stage. The preload device can be configured to apply a preload force to the wafer. In addition, two of the pins can be arranged to contact respective portions of a flat edge of the wafer, and a third pin and the preload device can be arranged to contact respective portions of a curved edge of the wafer. The third pin and the preload device can be arranged at respective locations on the stage to optimally constrain the wafer to the stage.
Additionally, the respective locations on the stage for the third pin and the preload device can optimize a restoring moment about an instantaneous center of rotation (ICR) imposed by the preload force on the wafer. Optionally, the respective locations on the stage for the third pin and the preload device can maximize the restoring moment about the ICR imposed by the preload force on the wafer. For example, the restoring moment can be maximized as a function of a first angle formed between an axis of the wafer and the respective location of the third pin.
Additionally, the ICR imposed by the preload force on the wafer can be an intersection point between a line of action associated with the third pin and a line of action associated with one of the two pins. A line of action can be defined by a respective direction of a reaction force at a pin. Alternatively or additionally, respective lines of actions associated with the two pins can be approximately parallel.
Alternatively or additionally, the pins can optionally be tapered.
Alternatively or additionally, the preload device can optionally be a flexural bearing.
Another example passive wafer alignment device can include a stage for holding a wafer, a plurality of tapered pins arranged on the stage, the tapered pins being arranged to contact respective portions of the wafer, and a preload device arranged on the stage. The preload device can be configured to apply a preload force to the wafer.
Additionally, the tapered pins can optionally be configured to constrain the wafer to the stage via surface-to-surface contact.
Yet another example passive wafer alignment device can include a stage for holding a wafer, a plurality of pins arranged on the stage, the pins being arranged to contact respective portions of the wafer, and a flexural bearing arranged on the stage. The flexural bearing can be configured to apply a preload force to the wafer.
Additionally, the flexural bearing can optionally define a circular contact interface.
Alternatively or additionally, the flexural bearing can optionally be further configured to adjust the preload force applied to the wafer.
An example method for producing a passive wafer alignment device can include arranging a plurality of pins on a stage for holding a wafer, determining respective optimal locations on the stage for a third pin and a preload device for applying a preload force to the wafer, and arranging the third pin and the preload device at the respective optimal locations on the stage. In addition, two of the pins can be arranged to contact respective portions of a flat edge of the wafer, and the third pin and the preload device can be arranged to contact respective portions of a curved edge of the wafer.
Additionally, the step of determining respective optimal locations on the stage for a third pin and a preload device can include analyzing a plurality of reaction forces at each of the pins as a function of a first angle formed between an axis of the wafer and the respective optimal location of the third pin and a second angle formed between the axis of the wafer and the respective optimal location of the preload device. Additionally, the step of determining respective optimal locations on the stage for a third pin and a preload device can further include determining values of the first angle and the second angle that result in compressive reaction forces at the pins.
Alternatively or additionally, the step of determining respective optimal locations on the stage for a third pin and a preload device can include analyzing a restoring moment about an instantaneous center of rotation (ICR) imposed by the preload force on the wafer. Optionally, the step of determining respective optimal locations on the stage for a third pin and a preload device can include maximizing the restoring moment about the ICR imposed by the preload force on the wafer. Optionally, the restoring moment can be maximized as a function of a first angle formed between an axis of the wafer and the respective optimal location of the third pin.
Alternatively or additionally, the ICR imposed by the preload force on the wafer can be an intersection point between a line of action associated with the third pin and a line of action associated with one of the two pins. A line of action can be defined by a respective direction of a reaction force at a pin. Alternatively or additionally, respective lines of actions associated with the two pins can be approximately parallel.
Alternatively or additionally, the pins can optionally be tapered. For example, the pins can be configured to constrain the wafer to the stage via surface-to-surface contact.
Alternatively or additionally, the preload device can be a flexural bearing. Optionally, the flexural bearing can define a circular contact interface.
Alternatively or additionally, the preload device can optionally be further configured to adjust the preload force applied to the wafer.
Other systems, methods, features and/or advantages will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features and/or advantages be included within this description and be protected by the accompanying claims.
To facilitate an understanding of and for the purpose of illustrating the present disclosure, example features and implementations are disclosed in the accompanying drawings, it being understood, however, that the present disclosure is not limited to the precise arrangements and instrumentalities shown, and wherein similar reference characters denote similar elements throughout the several views, and wherein:
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure. As used in the specification, and in the appended claims, the singular forms “a,” “an,” “the” include plural referents unless the context clearly dictates otherwise. The term “comprising” and variations thereof as used herein is used synonymously with the term “including” and variations thereof and are open, non-limiting terms. The terms “optional” or “optionally” used herein mean that the subsequently described feature, event or circumstance may or may not occur, and that the description includes instances where said feature, event or circumstance occurs and instances where it does not. This disclosure contemplates that the metrology devices and methods described herein can be used with nanoscale and/or microscale metrology applications such as atomic force microscopy, scanning tunneling microscopy, and/or nearfield optical scanning microscopy. The metrology devices and methods can be used to reduce setup time for nanoscale and/or microscale metrology applications.
In accordance with the present disclosure, a mechanical wafer alignment device (e.g., a passive wafer alignment device) is described. The wafer alignment device can include three pins and a nesting force applied by a preload device to keep a wafer in contact with the pins to restrain the wafer in six degrees of freedom. In certain embodiments, the kinematic coupling can be used to precisely mate the mechanical wafer alignment device below an XY precision stage that includes an array of Atomic Force Microscope (AFM) microchips. Using the mechanical wafer alignment device, one may achieve a wafer setup time of less than one minute and a lateral positioning accuracy on the order of 1 μm. In particular, the mechanical wafer alignment device disclosed herein can decrease setup times in AFM metrology. Translational positioning repeatability better than 2 μm and rotational repeatability better than 100 μrad can also be realized, for example. Sample setup times can be reduced to less than one minute, for example. The substantial time savings over traditional alignment methods as well as positioning repeatability well within the range of newly available AFM chips make the mechanical wafer alignment devices disclosed herein a suitable fixture for inline AFM metrology.
In accordance with the present disclosure, a method is also provided for determining optimal pin locations for three pins of a mechanical wafer alignment device. In particular, calculating optimal pin locations when using three pins to exactly constrain an object in one plane. Assuming that two pins are fixed with parallel lines of action, the method can determine suitable locations of the third pin and the nesting force (e.g., a preload force) with respect to the edge of the object in contact with the two pins with parallel lines of action. In accordance with the present disclosure, an optimal position of the third pin and an optimal position of the nesting force can be determined as function of a restoring moment imposed by the nesting force on the object. The restoring moment can be imposed about the finite instantaneous centers of the pin constraints with respect to each other. This moment can be, for example, a maximum magnitude and be in direction that would ensure that the object remains in contact with all three pins.
The method disclosed herein can be applied to the problem of, for example, passively aligning a wafer on a stage. The stage can comprise, for example, three pins and a preload device (e.g., a flexural bearing) that applies a nesting force (e.g., a preload force) in order to keep the three pins in contact with the wafer. Two of the pins in contact with the wafer can be located on the main flat of the wafer (also referred to herein as the “wafer flat”), for example. The method disclosed herein can be used to determine a suitable position for the third pin, as well as a suitable location of the preload device with respect to the wafer flat. The nesting force is applied by a flexural bearing which was designed to minimize jamming between the wafer and two pins.
Referring now to
The kinematic coupler 300 can optionally be configured to constrain the first stage 100 in six degrees of freedom. The kinematic coupler 300 can optionally constrain the XY precision stage 100 in the fixed position relative to the sample stage 200 using a magnetic force (e.g., using magnets) and/or using the force of gravity. This disclosure contemplates that the kinematic coupler 300 can include at least one fastener and a corresponding groove, where the fastener interfaces with the corresponding groove such that the XY precision stage 100 is constrained in the fixed position relative to the sample stage 200.
The metrology device can also optionally include a plurality of micrometers 400. The micrometers 400 can be attached to the XY precision stage 100. Additionally, the micrometers 400 can be configured to adjust the position of the XY precision stage 100 relative to the sample stage 200. The micrometers can be used to make fine adjustments to the position of the XY precision stage 100 relative to the sample stage 200. For example, one or more micrometers can be used to adjust the position of the XY precision stage 100 relative to the second stage 200 in an out-of-plane direction (e.g., the Z-direction or Z-axis translation). As described below, in some implementations, the XY precision stage 100 defines a two-dimensional plane (e.g., the X-Y plane). Accordingly, displacements or translations in an X-direction and/or a Y-direction are referred to herein as the in-plane directions and displacements or translations in a Z-direction, which are orthogonal to the X- and Y-directions, are referred to as the out-of-plane direction. Alternatively or additionally, one or more micrometers can be used to adjust X-axis or Y-axis rotation. Optionally, after a fixed position is established (e.g., using a first sample), the micrometers 400 can be locked, and the XY precision stage 100 can be repeatedly removed from and returned to the fixed position relative to the sample stage 200 using the kinematic coupler 300.
Referring now to
In some implementations, the pins 210 can optionally be tapered, for example. For example, the pins 210 can be configured to constrain the wafer to the stage via surface-to-surface contact. An example tapered pin 210 is shown in
Referring again to
In some implementations, as described herein, the third pin 210c and the preload device 225 can be arranged at respective locations on the stage 200 to optimally constrain the wafer to the stage. As described herein, the respective locations on the stage 200 for the third pin 210c and the preload device 225 can optimize a restoring moment about an instantaneous center of rotation (ICR) imposed by the preload force on the sample 250 (e.g., a wafer). ICRs are described below with regard to
Referring now to
This disclosure contemplates that the optimal locations can be determined (e.g., step 404 of
Alternatively or additionally, the step of determining respective optimal locations on the stage for a third pin and a preload device can include analyzing a restoring moment about an ICR imposed by the preload force on the wafer. ICRs are described below with regard to
Alternatively or additionally, the ICR imposed by the preload force on the sample 250 (e.g., a wafer) can be an intersection point between a line of action associated with the third pin 210c and a line of action associated with one of the two pins 210a and/or 210b. A line of action can be defined by a respective direction of a reaction force at a pin (e.g., as shown in
Design and Optimization
A wafer in contact with a flat surface has three degrees of freedom—two translational and one rotational. Out of plane rotations and translation are thought to be sufficiently limited by the gravitational force on the wafer. Three pin constraints can be used to exactly constrain the wafer such that it has zero degrees of freedom. Two pins with intersecting lines of action (LOAs) constrain the translational degrees of freedom of the wafer. A third pin is used to constrain rotation of the wafer and its location is not trivial. Rigid bodies rotate about instantaneous centers located at the intersections of the LOAs of constraints. Thus, in order to constrain a rotational degree of freedom, two of the three constraints must be parallel, thereby forming an instantaneous center at infinity that prevents rotation. Conventional wafers possess a flat (e.g., the wafer flat or a flat edge of the wafer) which provides a convenient location for the constraints with parallel LOAs.
A nesting force (e.g., a preload force) can be applied by any suitable force applicator (e.g., a preload device such as a flexure bearing) to maintain contact between the wafer and the pins. As described above, a flexure bearing can be used to apply the preload force or nesting force. Using a flexural bearing to apply preload force can reduce the potential for jamming through design of the flexure and the use of a circular contact interface between the flexure and the wafer. The flexural bearing described herein can be mounted to ground via screws in slots, which allow for adjustment of the preload force applied to the wafer so that a known preload force can be precisely and repeatedly applied to the wafer.
Referring now to
In accordance with the present disclosure, an example method for optimizing constraint locations of pins can include analyzing reaction forces (e.g., fr, fl, f3) and moments as a function of the unknown angles θ and ϕ. Reaction forces at each pin can be determined, for example, using matrix inversion to satisfy static equilibrium of forces in the x-y plane and equilibrium of moments about the z-axis. Exemplary reaction forces at each of the three pins are shown in Equation 1 below.
Allowable combinations of ϕ and θ for a given wafer geometry can include those resulting in compressive reaction forces (e.g., positive values). Compressive reaction forces are needed to maintain the sample 250 (e.g., a wafer) in static equilibrium. From this constraint it is evident that for three pin environments, ϕ and θ must be on opposite sides of an imaginary vertical line drawn between the left pin 210b and the right pin 210a.
The intersections between pin lines of action constitute wafer instantaneous centers of rotation (ICRs). A line of action can be defined by a respective direction of a reaction force at a pin (e.g., defined by the directions of fr, fl, f3 as shown in
The nesting force moment is shown to be a function of (a) the magnitude of the nesting force, (b) the distance between the pins located on the wafer flat (e.g., pins 210a, 210b in
The first angle (e.g., θ) between the third pin 210c location and the x-axis can be bound to the region between, for example, about −45° and 80°. The lower bound can be used to prevent the third pin 210c from interfering with the right pin 210a. The upper bound can be used to prevent over-constraint in the y-direction by, for example, maintaining a clearance between the vertical coordinate of the top of the sample 250 and the contact point between the sample 250 and the third pin 210c. The lower bound for the nesting force angle (e.g., ϕ) can be, for example, about 100° to prevent over-constraint, and the upper bound for the nesting force angle can be, for example, about 315° in order to prevent interference with the left pin 210b. Limiting the resulting moments to those with the appropriate orientation as shown in
It should be appreciated that the logical operations described herein with respect to the various figures may be implemented (1) as a sequence of computer implemented acts or program modules (i.e., software) running on a computing device (e.g., the computing device described in
Referring to
In its most basic configuration, computing device 700 typically includes at least one processing unit 706 and system memory 704. Depending on the exact configuration and type of computing device, system memory 704 may be volatile (such as random access memory (RAM)), non-volatile (such as read-only memory (ROM), flash memory, etc.), or some combination of the two. This most basic configuration is illustrated in
Computing device 700 may have additional features/functionality. For example, computing device 700 may include additional storage such as removable storage 708 and non-removable storage 710 including, but not limited to, magnetic or optical disks or tapes. Computing device 700 may also contain network connection(s) 716 that allow the device to communicate with other devices. Computing device 700 may also have input device(s) 714 such as a keyboard, mouse, touch screen, etc. Output device(s) 712 such as a display, speakers, printer, etc. may also be included. The additional devices may be connected to the bus in order to facilitate communication of data among the components of the computing device 700. All these devices are well known in the art and need not be discussed at length here.
The processing unit 706 may be configured to execute program code encoded in tangible, computer-readable media. Tangible, computer-readable media refers to any media that is capable of providing data that causes the computing device 700 (i.e., a machine) to operate in a particular fashion. Various computer-readable media may be utilized to provide instructions to the processing unit 706 for execution. Example tangible, computer-readable media may include, but is not limited to, volatile media, non-volatile media, removable media and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. System memory 704, removable storage 708, and non-removable storage 710 are all examples of tangible, computer storage media. Example tangible, computer-readable recording media include, but are not limited to, an integrated circuit (e.g., field-programmable gate array or application-specific IC), a hard disk, an optical disk, a magneto-optical disk, a floppy disk, a magnetic tape, a holographic storage medium, a solid-state device, RAM, ROM, electrically erasable program read-only memory (EEPROM), flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices.
In an example implementation, the processing unit 706 may execute program code stored in the system memory 704. For example, the bus may carry data to the system memory 704, from which the processing unit 706 receives and executes instructions. The data received by the system memory 704 may optionally be stored on the removable storage 708 or the non-removable storage 710 before or after execution by the processing unit 706.
It should be understood that the various techniques described herein may be implemented in connection with hardware or software or, where appropriate, with a combination thereof. Thus, the methods and apparatuses of the presently disclosed subject matter, or certain aspects or portions thereof, may take the form of program code (i.e., instructions) embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other machine-readable storage medium wherein, when the program code is loaded into and executed by a machine, such as a computing device, the machine becomes an apparatus for practicing the presently disclosed subject matter. In the case of program code execution on programmable computers, the computing device generally includes a processor, a storage medium readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and at least one output device. One or more programs may implement or utilize the processes described in connection with the presently disclosed subject matter, e.g., through the use of an application programming interface (API), reusable controls, or the like. Such programs may be implemented in a high level procedural or object-oriented programming language to communicate with a computer system. However, the program(s) can be implemented in assembly or machine language, if desired. In any case, the language may be a compiled or interpreted language and it may be combined with hardware implementations.
Prototype Design
A prototype passive wafer alignment device as shown in
Experimental Setup
The passive wafer alignment devices and methods provided here have many advantages over conventional technology. For example, the method provided herein can be used to effortlessly determine the ideal locations for a pin not on a flat and locations for a corresponding preload force (e.g., nesting force). The resulting pin and nesting force locations have been experimentally proven to improve the repeatability of wafer positioning.
Referring now to
Optimal repeatability was achieved with a coupling configuration that maximized the moments about the instantaneous centers of rotation. As shown in
Thus, as described above, disclosed herein are passive wafer alignment devices and methods for improving the repeatability of a passive alignment mechanism, which can be embodied as a silicon wafer alignment device. While the example passive alignment device disclosed herein can be used to successfully align 100 mm silicon wafers, the method itself is an advancement of principles in exact constraint design. As such, the passive wafer alignment devices and methods disclosed herein can be used to align other suitable sizes of wafers and can also be useful for designing fixtures for other suitable mechanisms. The passive wafer alignment devices and methods disclosed herein can be applied to other systems, for example, a system designed to constrain a cylindrical object with a flat. The passive wafer alignment devices and methods can also be used to maximize the repeatability of a pin-based fixture constraining of any object so long as two pins are in contact with an edge on the object.
Advantages
Conventional alignment technologies mention that the third pin can be located so that the nesting force produces a substantial restoring moment about the instantaneous centers of the pins. In addition, literature on the subject of alignment suggests that the moment of the nesting force about each instantaneous center formed by two pins should be in the direction of the remaining pin. However, neither the literature nor conventional technology provide any detail on the precise location of the third pin or the nesting force, and much less a method for making such a determination.
The passive wafer alignment devices and methods disclosed herein use an algorithm that determines the exact location at which the nesting force moment is maximized and is in the proper direction. The technique for determining the location of the pins and the nesting force is unique in that it provides a single solution for both the location of one pin (e.g. The third pin) and the nesting force (e.g., the preload device).
The passive wafer alignment devices and methods disclosed herein has many advantages over conventional technology. For example, the algorithm described herein can be used to effortlessly determine the ideal locations for the pin not on the wafer flat (e.g., the third pin) and the nesting force (e.g., the preload device). The algorithm can also be applied to different applications and is limited only by the condition that two of three restraining pins lie on a straight edge of the object to be restrained. The resulting pin and nesting force locations have been experimentally proven to improve the repeatability of wafer positioning.
Another advantage of the passive wafer alignment devices and methods disclosed herein over conventional technologies is that it prevents significant sources of misalignment errors before they happen. The design uses a flexural bearing or spring to apply the nesting force or preload force. The flexure is adjustable so that the force applied to the wafer can be known with certainty. This prevents errors such as the wafer shattering from excessive force or inconsistencies in position due to varying deflections in the wafer pin contact interface. The double parallelogram flexure design also ensures that the force is consistently applied perpendicular to the flat face of the wafer. This prevents jamming and out of plane misalignment.
Another advantage of the passive wafer alignment devices and methods disclosed herein is the use of tapered pins a. These pins assist in preventing out of plane alignment that other technologies do not consider. The passive wafer alignment devices and methods disclosed herein address this problem of conventional technologies by constraining all degrees of freedom of the wafer. It is neither over constrained nor under constrained. The mechanism is designed to reduce the chances of becoming over or under constrained. In addition, the mechanism corrects small initial alignment errors when the wafer is placed on the stage so that the wafer always has a tendency to be in contact with three pins.
Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.
This application claims the benefit of U.S. Provisional Patent Application No. 62/250,468, filed on Nov. 3, 2015, entitled “SYSTEMS AND METHODS FOR PASSIVE ALIGNMENT OF SEMICONDUCTOR WAFERS,” the disclosure of which is expressly incorporated herein by reference in its entirety.
This invention was made with government support under Grant no. EEC1160494 awarded by the National Science Foundation. The government has certain rights in the invention.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/US2016/060236 | 11/3/2016 | WO | 00 |
Number | Date | Country | |
---|---|---|---|
62250468 | Nov 2015 | US |