Process for monitoring measuring device performance

[0001] 1. Field of the Invention

[0002] The present invention relates generally to calibrating test equipment and, more particularly, to a method of maintaining calibration of scanning electron microscopes (SEMs) such that repeatable performance may be obtained.

[0003] 2. Description of the Related Art

[0004] This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

[0005] The scanning electron microscope (SEM) is used to create magnified images. As with all microscopes, the main purpose is magnification with clarity. Conventional light microscopes use multiple glass lenses to bend light waves to create a magnified image. The SEM is able to achieve much higher magnification than the light microscope largely because the SEM uses electrons instead of light waves to create the image. The SEM emits a beam of high energy electrons through a column in which a vacuum has been established. This beam travels downward and is focused into a very fine point by a series of magnetic lenses. The focused beam moves back and forth across the specimen, scanning it row by row. As the electron beam contacts the specimen at each spot, secondary electrons are displaced from its surface and electrons are scattered. These electrons are then counted by a detector, which sends signals to an amplifier. The number of electrons emitted from each spot on the sample is determined, and the data is combined to create the final image. The amount of scattering of electrons in any direction is a function of the angle made by the beam with the surface of the specimen and as a result, the image produced has a three dimensional appearance.

[0006] One use of SEMs is in the manufacture of semiconductor devices. SEMs are typically used in semiconductor manufacturing to measure the dimensions of articles of manufacture to ensure that certain specifications are met. This primarily involves measuring what is known as the critical dimension (CD) of an object such as an integrated circuit die or feature thereof. The CD is generally the length or width of a line, space, or contact. When the CD of a particular article is outside of a defined range, that article may not meet required specifications. If, for example, a wafer has been over exposed, an electrical contact may become enlarged such that it is beyond the specification for that part. This error can be made evident by utilizing an SEM to measure the CD. When a product is found to be defective for having a measurement that is out of specification, the product may be scrapped or subjected to corrective measures.

[0007] An SEM must be manually adjusted on a regular basis to keep it at optimum performance. These manual adjustments involve adjustment of resolution, magnification, and lens aberration. Because these adjustments are subjective, variations can be introduced resulting in distortion and incorrect measurement. For example, magnification in one part of the image may be different from another part of the image. The undesirable variation in image magnification may result in errors in categorizing devices measured with the SEM. In other words, devices measured with the SEM may be incorrectly categorized as out of specification. More problematic is the possibility that devices that in fact do not meet a given specification could be categorized as meeting the specification. Erroneous categorization of devices that are actually out of specification could result in subsequent problems such as failure of a device in the field or other performance problems.

[0008] A known method of calibration involves measuring a line and using the measurements to judge subsequent SEM setups as either acceptable or unacceptable. After the initial measurements of the line are obtained, the same line may be re-measured at periodic intervals (for example, daily or weekly). If measurements taken at later times are within a predetermined tolerance of the original measurements, the setup of the device may be deemed to be acceptable. If subsequent measurements are outside of a predetermined range, the machine may be subject to corrective action, such as modifying certain settings.

[0009] One problem with using lines as setup guides is that carbon deposits may accumulate on the line to be measured and actually change the length of the line. Thus, subsequent measurements of the line may not accurately reflect the true calibration of the SEM. SEMs are typically used to measure very small structures, so even small buildups may be problematic.

[0010] Another known practice for calibration involves what is known as a pitch test. In a pitch test, the pitch between a space and a line are measured. The pitch (or orientation) of the space relative to the line has been found not to change significantly even if deposits are present. Accordingly, the pitch test somewhat overcomes the problem of carbon deposits. However, the pitch test method does not take into account that astigmatism may have been introduced into the setup of the SEM. Corrections to account for astigmatism may be performed through a process known as “wobbling.” In wobbling, an operator may wobble the lens of the SEM to attempt to stabilize the image. Wobbling, however, is prone to subjective error.

[0018] One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

[0019] Turning now to the drawings, FIG. 1 is a block diagram of a scanning electron microscope (SEM) 10. The SEM 10 comprises a vacuum column 20, which contains elements of the SEM 10 and allows air to be pumped out to create a vacuum. Typically, when an SEM 10 is in use, air is pumped out of the vacuum column 20, and an electron gun 30 situated at one end of the vacuum column 20 emits a beam of high energy electrons 40. The high energy electron beam 40 travels axially through the vacuum column 20, which contains a series of magnetic lenses 50 comprising condensing lenses 60 and an objective lens 70. These magnetic lenses 50 are designed to focus the electron beam 40 to a very fine spot.

[0020] Near the end of the vacuum column 20 opposite the electron gun 30, a set of scanning coils 80 function to move the focused electron beam 40 in a scanning motion, row by row, across a target specimen 90. Secondary electrons 100 are displaced from the surface of the target specimen 90 as the electron beam 40 hits each spot on the surface. A detector 110 counts the secondary electrons 100 and sends signals to an amplifier 120. Data from the amplifier 120 is converted into an image 130. The numbers of electrons emitted from each spot on the target specimen are used to build the final image 130, which may be displayed on a monitor 140.

[0021] In one example of the method disclosed herein, an original shape 200 is used as the target specimen 90 and is standardized to assist in calibration of the SEM 10. FIG. 2 depicts an original shape 200 as viewed through the SEM 10. The method disclosed herein may use a shape such as the one illustrated in FIG. 2 to assist with calibration of a device such as an SEM. As can be appreciated by those skilled in the art, a variety of shapes may be used.

[0022] The original test shape 200 may be made from material such as silicon or any other material. The material used to construct the original shape 200 is not believed to be a crucial aspect of the present invention. The original shape 200 may be measured repeatedly over a period of time to obtain data that may be used to create a mathematical model using techniques such as polynomial curve fitting or Fourier analysis. The data thus created may comprise master measurement data that may be used for subsequent comparison to ensure correct calibration of the SEM 10.

[0023] Closed shapes, such as the shape illustrated in FIG. 2, have been found to facilitate the creation of mathematical models that allow effective calibration of the SEM 10. This is true at least because mathematical modeling of closed shapes may produce data that corresponds to a wide variety of characteristics about the shape being modeled. That data may be used to correct specific features of the SEM setup that relate to the relevant characteristics without disturbing setup parameters that are unrelated. For example, a polynomial equation may be used to model the test shape mathematically when measurements are taken. Each coefficient of a polynomial equation may embody given information about a different characteristic of the measured object. Examples of the characteristics that may be embodied in a mathematical model include linearity, rotation, orientation, and the like. When a subsequent calibration measurement of a test shape is taken, the resulting mathematical model may indicate a variation in the parameters affecting the coefficient that represents rotation of the image. The elements of the SEM setup that affect the rotation of the image may then be adjusted to obtain better calibration.

[0024] As previously noted, a mathematical model of the original shape 200 is derived. FIG. 3 depicts a binary fit 300 of the original shape 200, which is essentially an outline of the original shape 200. In one example of this process, the binary fit 300 may be used to assist with mathematically modeling the original shape 200. Measurements of radii 310a, 310b and 310c of the original shape 200 and measurements of corresponding angles 320b and 320c may be obtained from the binary fit 300. The radii 310a, 310b and 310c and angles 320b and 320c illustrated in FIG. 3 are merely representative. Those of ordinary skill in the art will appreciate that there are actually many different radii and angles that are not specifically illustrated.

[0025] Additionally, depending on the shape being used, the various radii and angles may vary not only between shapes but also may vary at different points within a particular shape. These measurements may be used in one example of the method disclosed herein to establish a mathematical model of the original shape 200. For example, FIG. 4 depicts a polynomial fit 400 derived by graphing the varying radii of the original shape 200 on a Y-axis with the corresponding angles in radians on an X-axis. The particular polynomial fit 400 as shown is an eighth order polynomial which may be represented by the equation P(x) =a_nxⁿ+ . . . +a₂x²+a₁x¹+a₀, where n=8. The variable n represents the order of the polynomial. The higher the order of polynomial, the closer it will be to an exact mathematical description of the original shape and the more information will be obtainable from coefficients to assist with calibration. Also, higher order polynomial models yield more coefficients, each corresponding to a particular characteristic of the original shape 200. Higher order polynomial models thus produce more data and may result in more accurate calibration of the SEM 10.

[0026] As set forth above, a wide variety of mathematical modeling techniques may be used to create master measurement data. The order of polynomial used to model the original shape 200 and even the use of a polynomial type expression for representing the original shape are not believed to be crucial aspects of the invention. As can be appreciated by those skilled in the art, many mathematical equations adequately modeling the original shape 200 may be used.

[0027] The master measurement data obtained during an initial calibration may be used as a standard against which to judge subsequent calibration measurements. A mathematical model of the image as being viewed at the time of calibration is derived and compared to the mathematical model of the established standard. Differences in the two equations may indicate that certain corrections should be made to ensure the SEM 10 is properly calibrated within certain specifications. When variations between the master measurement data and data taken in a subsequent reading are found, the variations in the mathematical model data may indicate the specific areas of the SEM 10 that require adjustment.

[0028] For example, if a Fourier Series represented by the equation r(θ)=a₀+a₁ sin θ+b₁ cos θ+a₂ sin 2θ+b₂ cos 2 θ+ . . . +a_n sin nθ+b_n sin nθ is used to mathematically model the original shape 200, and the zero term, a₀, has changed between the standard mathematical model and the mathematical model derived at the time of calibration, this may indicate that the image has changed size. This could indicate a problem with the SEM settings that relate to magnification. Similarly, if the sin θ term changed, a change in symmetry may be indicated. This may indicate a problem with SEM settings that relate to orientation or rotation of the image. By knowing what terms have changed between the mathematical model of the image being taken and the mathematical model of the standard, one skilled in the art can relate those changes to what physical settings should be changed on the SEM 10 to properly calibrate the device.

[0029] FIG. 5 is a process flow diagram showing an example of a process for using an original shape to create a set of master measurement data that may be used for calibration of a measurement device. The technique is generally referred to by the reference numeral 500. At block 502, the technique begins. The beginning of the process may correspond to the powering up or initialization of a device such as the SEM 10.

[0030] At block 504, an original shape 200 is placed on a device, such as the SEM 10, for viewing. In one example of this technique, an original shape 200 is made from a material such as silicon. As shown at block 506, the dimensional measurements of the original shape 200 are recorded. Using the dimensional measurements, a mathematical model of the original shape 200 is derived, as indicated at block 508. The mathematical model is used to produce a set of master measurement data. As set forth above, the master measurement data may be comprised of measurement data taken over an extended period of time. At block 510, the process of creating the master measurement data ends. The master measurement data may be used as a standard by which to judge subsequent measurement data. The difference between the subsequent measurement data and the master measurement data may be used to calibrate the setup of the SEM 10.

[0031] FIG. 6 is a process flow diagram showing an example of a process for creating current measurement data and comparing the current measurement data to the master measurement data obtained from the process shown in FIG. 5. The process is generally referred to by the reference numeral 600. At block 602, the process begins. The beginning of the process may correspond to the powering up or initialization of a device such as the SEM 10. At block 604, the original shape 200 previously used to create the master measurement data is placed on a device for viewing.

[0032] As shown at block 606, the current dimensional measurements of the original shape 200 are recorded. Using the current dimensional measurements, a mathematical model of the original shape 200 is derived and current measurement data is produced, as illustrated at block 608. At block 610, the current dimensional data is compared to the master measurement data created, for example, using the process illustrated in FIG. 5. At block 612, a determination is made about whether the current dimensional data is close enough to the master measurement data to declare the calibration of the SEM 10 to be acceptable. One method of making this determination is to evaluate whether the current measurement data is within a predetermined range of the master measurement data.

[0033] If the current measurement data is not sufficiently close to the master measurement data, certain aspects of calibration corresponding to differences in the mathematical models are adjusted, as shown at block 616. Operation then continues at block 606. If, however, the current measurement data is sufficiently close to the master measurement data at block 612, the calibration of the SEM 10 is complete and the process ends, as shown at block 618.

[0034] While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.