Patent Publication Number: US-7724375-B1

Title: Method and apparatus for increasing metrology or inspection tool throughput

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of priority of U.S. provisional application No. 60/895,010 to Alex Novikov et al., entitled “METHOD AND APPARATUS FOR INCREASING METROLOGY OR INSPECTION TOOL THROUGHPUT”, filed Mar. 15, 2007, the entire disclosures of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   Embodiments of the invention generally relate to a method and apparatus for metrology and inspection measurements and more particularly to a method and apparatus for increasing throughput of the metrology measurements of the semiconductor wafer. 
   BACKGROUND OF THE INVENTION 
   As integrated circuit device geometries continue to shrink, manufactures have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. Techniques of this type, known generally as optical metrology and inspection, operate by focusing an optical beam from a tool on a portion of a sample and then analyzing the reflected or scattered energy. A higher level of throughput optical system is required in semiconductor manufacture. 
   The existing optical metrology and inspection methods require about 150 milliseconds for focusing after positioning the tool over a target. Throughput improvement of the existing methods with single head tools puts high demands on different system components. Methods for throughput improvement in an optical inspection system with a single head include shorter move time, shorter target acquisition time and quicker target measurement time. In addition, wafer inspection strategies for design rules smaller than 45 nanometers require a significant increase in the number of measurements per die and per wafer. However, the existing methods cannot provide the desired step function improvement. 
   It is within this context that the embodiments of the present invention arise. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: 
       FIG. 1  is a schematic diagram illustrating an optical system according to an embodiment of the present invention. 
       FIGS. 2A-2B  depict the footprints of the single tool system of the prior art and multiple tools system of the present invention. 
       FIG. 3  is a diagram illustrating a tool utilizing a focus mechanism of the prior art. 
       FIG. 4  is a diagram showing a tool utilizing a focus system according to an embodiment of the present invention. 
       FIG. 5  is a diagram showing a second focus system according to an alternative embodiment of the present invention. 
       FIG. 6  is a diagram illustrating a focus system according to another alternative embodiment of the present invention. 
       FIG. 7  is a block diagram showing a computing system that may be used in connection with facilitating employment of embodiments of the invention. 
   

   DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
   Although the following detailed description contains many specific details for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the exemplary embodiments of the invention described below are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention. 
     FIG. 1  is a schematic diagram illustrating an optical system  100  for performing metrology or inspection measurements on a test sample  106  such as a semiconductor wafer or a reticle. As shown in  FIG. 1 , the optical system  100  may comprise an apparatus  102  that includes a stage  108 , such as a wafer chuck, adapted to hold the test sample  106 . The apparatus  102  includes one or more metrology or inspection tools  116  supported by a supporting structure  118  and positioned proximate the stage  108  to make measurements of the test sample  106 . For the sake of example, two tools  116  are shown in  FIG. 1 . Each tool  116  may focus an incident beam of radiation or charged particles on the test sample  106  and detect radiation and/or charged particles that are scattered by, reflected by or otherwise generated by interaction between the incident beam and the test sample  106 . The inspection or metrology tools  116  are preferably of the same type, which include inspection tools, ellipsometers, reflectometers and the like. 
   A movement mechanism, which is not shown in  FIG. 1 , is adapted to move the supporting structure  118 , XY stage  110  and Z stage  112  such that either the test sample  106  or the tool(s)  116  move with respect to one another from a first position to a second position, for example, moving the test sample while the tool remains fixed or moving the tool while the test sample remains fixed. At the second position, the tool(s) is aligned for measurement of one or more measurement targets on the sample  106 . 
   The apparatus  102  also includes a focusing mechanism  104  positioned proximate one or more of the tools  116  and operably coupled to the tool(s)  116  to adjust a focus of the tool  116  on the test sample  106  during movement of the sample  106  and/or tool  116  from the first position to the second position. The optical system  100  may also include a controller  114  operably coupled to the focusing mechanism  104  to control the movement of the sample  106  and/or the tool(s)  116  in response to a signal from a detector incorporated in the focusing mechanism  104  in a direction parallel to an optical axis of the tool  116 . 
   The controller  114  may include coded instructions  132  that, when executed, cause the components of the system  100  to move at least one of the test sample  106  and the tool  116  with respect to one another from a first position to a second position at which the tool  116  is aligned for measurement of a measurement target on the sample  106 . During the movement from the first position to the second position, the instructions  132  cause the system  100  to adjust a focus of the tool  116  on the test sample. 
   By way of example, the tool may be initially brought into focus at the first position. During movement from the first position to the second position, the detector  104  may measure changes in the relative position between the tool  116  and the test sample  106  as measured along the z-direction that is perpendicular to x- and y-directions. The controller  104  may compensate for these changes by directing the z-stage  112  to adjust z-position of the test sample  106  in a way that maintains a constant distance between the tool  116  and the test sample  106  (as measured along the z-direction) during movement the first position to the second position. The tool-sample distance adjustment may be made continuously or intermittently during the movement. 
   It is also noted that, in some embodiments, the tool-sample distance may be partly or entirely adjusted along the z-direction using a positioning mechanism  117  that can move the tool  116  relative to the support structure  118  along the z-direction. By way of example, the positioning mechanism  117  may include a piezoelectric actuator, or other actuator, that is responsive to signals from the controller  114 . Preferably the range of movement provided by the z-stage  112  and/or positioning mechanism  117  is sufficient to accommodate for changes in focus across the substrate, e.g., due to variation in sample thickness, warping, tilt of the stage  108  and the like. 
   In certain embodiments of the present invention, the system  100  may use multiple tools. The advantage of using more than one tool is that the XY stage footprint of a tool may be reduced, e.g., as shown in  FIGS. 2A-2B .  FIG. 2A  illustrates a footprint of an optical system with only one tool  116 . For a test sample  106  with a diameter D, the footprint of single tool  116  system is approximately 2D in both X and Y directions. As shown in  FIG. 2B , the optical system with two tools  116  will reduce the footprint of each tool  116  in x direction by half, i.e., only 1D. If the optical system includes four tools the footprint may reduce by quarter compared with the optical system with single tool and so on. 
   A measurement requires very precise positioning of a measurement head over a measurement target. The tool positioning mechanisms  117  may adjust the position of each tool  116  relative to the sample  106  in the x-, y- and z-directions. In certain applications, the pitch between targets on a sample (e.g., die on a semiconductor wafer) may be constant over the sample  106 . In such a case, the tools  116  may be spaced from each other by a certain distance such that each tool can be positioned over its target with the required accuracy. The tool-to-tool distance may be adjusted, e.g., using the tool positioning mechanisms  117  and appropriate signals from the controller  104 . Adjustable positioning of each tool  116  allows compensation for small target-to-target position deviations from the constant pitch. Moreover, such tool-to-tool spacing adjustment allows the system  100  to support different target pitches according to the recipe, the die size and the wafer mapping. 
   In multi-tool embodiments the tools  116  may be either of the same type or different types. In certain multi-tool embodiments, the tools  116  are preferably same type and the distance between the tools is adjustable at run time. Active adjustment of the tool-to-tool pitch allows fitting to the die size or the grid size of the features to be measured. Embodiments that use more than one tool  116  may measure more than one measurement targets on the sample  106  at the same time and may reduce the number of moves, the target acquisition time, and the total time spent taking measurements, all of which can increase the tool throughput. 
   It is noted that there are other ways of adjusting the focus of the tools  116 . For example, the tools  116  may each include an optical column adapted to focus light or charged particles (e.g., electrons) on the sample  106 . Such optical columns may include one or more lenses. In some embodiments, the positioning mechanism may adjust the positioning of one or more such lenses within the tool  116  to change the focus. Alternatively, the positioning mechanism  117  may adjust the optical properties of one or more lenses within the tool  116  without having to move them. For example, in the case of a tool having charged particle optical column, such as an electron beam column, the lenses might work by either electrostatic or electromagnetic focusing. In such a case, the beam positioning mechanism  117  may be an electrical power supply that adjusts either a voltage applied to an electrostatic lens or a current applied to a coil in an electromagnetic lens to change the focus of the tool  116 . 
   In order to implement focusing “on-the-fly”, e.g. continues focus while wafer is moving, in embodiments of the present invention, certain changes may be made compared to focusing systems used in the prior art.  FIG. 3  illustrates focus sensing as implemented in a prior art imaging tool  300 . The focus sensing is typically based on a scanning interferometer  301 , such as a Michelson or Linnik-type interferometer that is built into the tool. A light source  302  is optically coupled to the interferometer  301 . The light source  302  may be a laser, LED, thermal source, or other optical source. To enhance robustness, the light source  302  may be a broadband source, such as a thermal source. Illumination light from the light source  302  is divided by a main beam splitter  312  to define test light and reference light. The test light is reflected by the splitter  312  and focused by a main objective  318  onto a test sample  320 . The main objective  318  has an exit pupil  328 . Similarly, the reference light transmitted by the main beam splitter  312  is focused by a reference objective  314  having an exit pupil  330 , onto a reference mirror  316 . Preferably, main objective  318  and reference objective  314  have similar optical properties. 
   Test light reflected (or scattered or diffracted) from the test sample  320  propagates back through the main objective  318 , is transmitted by the beam splitter  312 , and is imaged through a tube lens  310 . Similarly, reference light reflected from the reference mirror  316  propagates back to through the reference objective  314 , is reflected by beam splitter  312  and is imaged through the tube lens  310 . Test and reference lights from the tube lens  310  are partially transmitted by a focus beam splitter  308  and are imaged on an image sensor  303  such as a charge-couple device (CCD) array, where the test light and the reference light interfere with each other. 
   Test and reference light from the tube lens  310  are partially reflected by the focus beam splitter  308  and focused on a focus sensor  304  where they interfere with each other. The image sensor  303  and focus sensor  304  measure the intensity of the optical interference at one or more pixels as the relative position of the test sample  320  is scanned in the z-direction. The signals from the image sensor  303  and focus sensor  304  may be analyzed to adjust the focus of the tool  300 . 
   Although it may be possible to use the image sensor  303  to implement the function of the focus sensor  304 , it is often advantageous to provide a separate focus sensor. For example, focus sensor  304  may be a very fast photodiode sensor providing a sampling rate about 100 kHz, whereas the imaging sensor  303  may be a CCD camera with sampling rate about 30-60 Hz. The use of a more rapid focus sensor allows for a faster focus than would be otherwise attainable with the relatively slow CCD camera. 
   The prior art focusing system  300  may be used to focus on a specific target at a particular X-Y position when the focusing system is positioned above the target and stays in a fixed XY position during the focusing procedure. Since the optical system  300  provides images of the test sample  320  and reference mirror surface  316  on the image sensor  303 , the collected signal is formed as a result of interference between test and reference light beams corresponding to the different optical paths and achieves its maximum when both paths have the same optical length. The reference path length may be set at a length for which the sample is in focus. When the tool performs a scan in z-direction a complex interference signal is detected due to the target topography. Since the form of the detected signal strongly depends on the target design and the topography of the underlying layers the test sample  320  is required to be in a correct X-Y position with respect to the tool while the Z scan is performed for determining the correct focus. 
   The “focus on the fly” procedure of embodiments of the present invention reduces the overall time required for measurement by replacing the prior art focus mechanism with a continuous focus correction during site-to-site navigation.  FIG. 4  is a diagram showing an imaging tool  400  implementing focusing according to an embodiment of the present invention. The focus mechanism of the tool  400  is basically similar to prior art focus mechanism  300 . The main difference between the tool  400  and the prior art tool  300  is that the focus mechanism performs pupil imaging instead of object or field imaging as in the prior art tool  300 . To implement pupil imaging, the focus beam splitter  308  is located between the main beam splitter  312  and the tube lens  310  as shown in  FIG. 4  and the focus sensor  304  is conjugate to the exit pupils  328 , 330 . In addition, the focus sensor  304  may be an array of sensors, e.g., a CCD array or array of diode sensors that senses optical intensity across interfering images of the exit pupils  328 ,  330 . An advantage of the focus mechanism used in the tool  400  is that it requires relatively minor changes in the existing architecture and in this regard is much more attractive than other proposals. 
   When the distance between main objective lens pupil  328  and the test sample  320  is equal to the distance between the reference objective lens pupil  330  and the reference mirror  316 , the distribution of light intensity on the focus sensor  304  array is uniform or quasi-uniform (taking into account the dependence of reflection coefficient on angle of incidence). When the tool  400  is out of focus, the phase difference ΔΦ between the beam from the main objective lens  318  and the beam from the reference objective lens  314  can be written across the pupil as: 
             ΔΦ   =         4   ⁢   π     λ     ⁢   Δ   ⁢           ⁢   F   *     cos   ⁡     (   α   )           ,         
where ΔF is the deviation from correct in focus, α is the angle of incidence, and λ is the average wavelength of the light.
 
   The angle of incidence α is given by 
           α   =       sin     -   1       (           x   2     +     y   2         f     )           
where x and y are the pupil coordinates and f is the focal length of the objective lens. If the light intensities from the sample and the reference mirror are denoted as I s  and I r  the detected interference signal in these terms can be represented as I=I 1 +I 2 +2*√{square root over (I 1 I 2 )} cos(ΔΦ). For example when ΔF=λ/4 the intensity (I) in the center of CCD (pupil center) is I=I 1 +I 2 −2*√{square root over (I 1 I 2 )} and it is almost zero when I 1 =I 2 . Whereas the intensity at CCD (pupil) periphery for NA=0.7 is I=I 1 +I 2 −1.2*√{square root over (I 1 I 2 )} which is approximately a half of the sum of the light intensities from main and reference optical paths.
 
   If the tool moves away from the focus position, a number of interference fringes are detected by the image sensor  303 . One or more focus positions may be established and corresponding image patterns on image sensor may be recorded for these focus positions. Focus correction may then be implemented during navigation by analyzing deviations from these focus patterns during the navigation with a large number of discrete time events, e.g., proportional to the rate of data readout from the focus sensor  304 . In this respect the usage of photo diode detectors allows to increase the readout rate up to about 100 kHz. 
   The signal from the focus sensor  304  and image sensor  303  may be coupled to a controller  306  that controls adjustment of the focus of the tool  400 , e.g., by adjusting the position of the test sample  320  relative to a z-axis with a z-stage  324  as an x-y stage  326  scans the test sample in a direction more or less perpendicular to the z-axis. The sign of the focus deviation signal ΔF can be obtained by fast, small-amplitude modulations of the phase of the signal, and by observing in a phase-locked manner the phase of the focus deviation signal ΔF resulting from this modulation. Two examples of such a phase modulation methods are: i) modulating the position of the reference mirror  316 , e.g., with a piezoelectric transducer, at a high frequency and small amplitude during movement of the test sample  320 ; ii) modulating the focal position (z-position) of the test sample  320  while it is moving. 
   In order to eliminate losing the focus and to enable focus mechanism to follow the wafer topography a high spatial sampling rate is often desirable. For example, if the stage  324  moves with velocity of about 500 cm/s, a focus readout rate of 100 KHz would allow correction of the focus position for each 50 μm of distance traveled by the stage. 
     FIG. 5A  is a diagram of a focusing mechanism  500  according to an alternative embodiment of the present invention. The focusing mechanism  500  does not rely on imaging optics of a particular tool. The focus mechanism  500  includes a beam splitter  504 , an optical delay array  502 , a focus sensor  506  having an array of optical sensors  510  and a reference mirror  508 . By way of example, the optical delay array  502  may be a stepped glass plate, glass wedge, or an array of EO-modulators. In this focus mechanism, each optical beam from each optical delay path is detected by one sensor  510  of focus sensor array  506 . 
   Collimated light from a light source  501  (e.g., a laser) is split by beam splitter  504  into reference and test beams. The reference beam passes through the optical delay array  502 , while the test beam is reflected from the sample  320 . The focusing mechanism  500  is based on classical interferometer scheme where the amplitude of the signal from the detectors  510  changes with defocus as: 
             I   =       I   0     +       I   1     *     cos   ⁡     (           2   ⁢   π     λ     ⁢   Δ   ⁢           ⁢   F     +   ϕ     )             ,         
where φ is a known phase, ΔF is the focus deviation and
 
               2   ⁢   π     λ     ⁢   Δ   ⁢           ⁢   F         
is a phase variation with focus deviation.
 
   Since the values of I 0  and I 1  may change when the tool navigates above the wafer, in order to find the unknown phase change containing information about focus position, at least three different intensities corresponding to three different focus positions are selected. By selecting three or more areas and introducing an optical delay array  502  as shown in  FIG. 5 , a simple phase algorithm allows calculation of the phase φ and by this way to define the focus deviation ΔF. Such an algorithm may be implemented, e.g., as part of the code instructions  132  described above. 
   By way of example, in the illustrated example where there are four sensors  510  and the optical delay array  502  accommodates four different beams, the delay for the i th  beam may be defined as 
   
     
       
         
           
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   Using this known pattern of delays intensity variations measured by the sensors  510  due to change of focus may be separated from intensity variations due to changes in I 0  and I 1 . Specifically, intensity variations due to changes in I 0  and I 1  as measured by the sensors  510  will be independent of the delay. Intensity variations due to the phase φ will differ for the different delays by a calculable amount. 
   Signals from the sensors  510  may be coupled to a controller  306  that controls adjustment of the focus of a tool that uses the focus system  400 . By way of example, the controller may direct a z-stage  324  to adjust the position of the test sample  320  relative to a z-axis as an x-y stage  326  scans the test sample in a direction more or less perpendicular to the z-axis. The z-axis position of the sample  320  may be adjusted to minimize the focus deviation ΔF. A possible problem may appear when the focus is changed in a range larger than λ since the prediction for ΔF and ΔF±nλ may be the same. To solve this problem the focus system  400  may use a few wavelengths simultaneously. 
   The concept behind the focus system  500  may be implemented in an imaging tool in a manner similar to that shown in  FIG. 4 . By way of example, and without loss of generality,  FIG. 6  is a diagram of metrology or inspection tool  600  according to another alternative embodiment of the present invention. The tool  600  is basically similar to the tool  400 . The differences are that in the focus mechanism  600 , an optical delay array  604  is placed between the beam splitter  312  and the reference objective lens  314 . In addition, a focus sensor array  602  is a multiple detector array such as line-CCD, 2-dimensional CCD, photodiode array, or any other detector array capable of mapping a fringe pattern with sufficient spatial and temporal density. The position of the test sample  320  is probed with the interferometer, using n separate beams (e.g., beams that are spatially separate in the pupil  328  of the main objective  318  and pupil  330  of the  314  reference objective). Each test beam from the test sample  320  goes through the optical delay array  604 , defined by delay=iλ/n for the i th  beam and each test beam from each optical delay path is detected by one detector of the focus sensor array  602 . 
   Since the reflection amplitude and phase of the reflection coefficient at the sample surface depend on the angle of incidence of the light from the test beam path it is desirable to arrange all optical delay areas in the delay array  604  in a symmetric fashion with respect to rotation about the beam axis. The delays in the delay array  604  may be used to separate intensity variations due to changes in focus from intensity variations due to changes in reflection from the sample  320 , as described above with respect to  FIG. 5 . 
     FIG. 7  is a block diagram of an example of computing system  700  in connection with facilitating employment of the subject invention. With reference to  FIG. 7 , the computer system  700  includes a computer  701 . The computer  701  includes a processing unit  703 , a system memory  705 , and a system bus  727 . The system bus  727  couples system components including, but not limited to, the system memory  705  to the processing unit  703 . The processing unit  703  can be any of various available processors. 
   The system bus  727  can be any of several types of bus structure(s) including a memory bus or memory controller, a peripheral bus or external bus, and/or a local bus using any variety of available bus architectures including, but not limited to, 11-bit bus, Industrial Standard Architecture (ISA), Micro-Channel Architecture (MSA), Extended ISA (EISA), Intelligent Drive Electronics (IDE), VESA Local Bus (VLB), Peripheral Component Interconnect (PCI), Universal Serial Bus (USB), Advanced Graphics Port (AGP), Personal Computer Memory Card International Association bus (PCMCIA), and Small Computer Systems Interface (SCSI). 
   The system memory  705  may contain the coded instructions  132  described above. The system memory  705  may include volatile memory and/or nonvolatile memory. The basic input/output system (BIOS), comprising the basic routines to transfer information between elements within the computer  701 , such as during start-up, may be stored in nonvolatile memory. By way of illustration, and not limitation, nonvolatile memory can include read only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable ROM (EEPROM), or flash memory. Volatile memory includes random access memory (RAM), which acts as external cache memory. By way of illustration and not limitation, RAM is available in many forms such as synchronous RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDR SDRAM), enhanced SDRAM (ESDRAM), Synchlink DRAM (SLDRAM), and direct Rambus RAM (DRRAM). 
   The computer  701  may optionally include removable/non-removable, volatile/non-volatile computer storage media  709 , for example a disk storage. Storage medium  709  may includes, but is not limited to, devices like a magnetic disk drive, floppy disk drive, tape drive, Jaz drive, Zip drive, LS-100 drive, flash memory card, or memory stick. In addition, storage medium  709  can include storage media separately or in combination with other storage media including, but not limited to, an optical disk drive such as a compact disk ROM device (CD-ROM), CD recordable drive (CD-R Drive), CD rewritable drive (CD-RW Drive) or a digital versatile disk ROM drive (DVD-ROM). To facilitate connection of the disk storage devices  709  to the system bus  727 , a removable or non-removable interface is typically used such as interface  707 . 
   The computer system  700  may also includes input devices  719  such as a pointing device such as a mouse, trackball, stylus, touch pad, keyboard, microphone, joystick, and the like. These and other input devices may connect to the processing unit  703  through the system bus  727  via interface port(s)  713 . Interface port(s)  713  include, for example, a serial port, a parallel port, a game port, and a universal serial bus (USB). Output device(s)  717  use some of the same type of ports as input device(s)  719 . Thus, for example, a USB port can be used to provide input to the computer  701  from the focusing mechanism  104 , and to output information from computer  701  to an output device  717  or to the z-stage  112 . Output adapter  711  is provided to illustrate that there are some output devices  717  like monitors, speakers, and printers, among other output devices  717 , which may require special adapters. The output adapters  711  may include, by way of illustration and not limitation, video and sound cards that provide a means of connection between the output device  717  and the system bus  727 . 
   The computer system  700  may also include a network interface  721  to enable the device to communicate with other the focusing mechanism  104  and/or other devices over a network, e.g., a local area network or a wide area network, such as the internet. Communication connection  715  refers to the hardware/software employed to connect the network interface  721  to the bus  727 . While communication connection  715  is shown for illustrative clarity inside computer  701 , it can also be external to computer  701 . The hardware/software necessary for connection to the network interface  721  includes, for exemplary purposes only, internal and external technologies such as, modems including regular telephone grade modems, cable modems and DSL modems, ISDN adapters, and Ethernet cards. 
   While the above is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. Any feature, whether preferred or not, may be combined with any other feature, whether preferred or not. In the claims that follow, the indefinite article “A”, or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for.”