The disclosure generally relates to the field of measuring technology, particularly to a method and apparatus for measuring the shape, the shape slope, the shape curvature or the film stress, and thickness variation of a wafer.
Thin polished plates such as silicon wafers and the like are a very important part of modern technology. A wafer, for instance, refers to a thin slice of semiconductor material used in the fabrication of integrated circuits and other devices. Other examples of thin polished plates may include magnetic disc substrates, gauge blocks and the like. While the technique described here refers mainly to wafers, it is to be understood that the technique also is applicable to other types of polished plates as well. The term wafer and the term thin polished plate may be used interchangeably in the present disclosure.
Generally, certain requirements may be established for the flatness, the shape as well as its derivatives, and thickness uniformity of the wafers. There exist a variety of techniques to address the measurement of shape, shape slopes, shape curvatures, and thickness variation of wafers. One such technique is disclosed in U.S. Pat. No. 6,847,458, which is capable of measuring the surface height on both sides and thickness variation of a wafer. It combines two phase-shifting Fizeau interferometers to simultaneously obtain two single-sided distance map between each side of a wafer and corresponding reference flats, and computes thickness variation and shape of the wafer from the data and calibrated distance map between two reference flats.
The present disclosure is directed to an interferometer system. The interferometer system includes a holding mechanism configured to hold a polished opaque plate substantially vertically, first and second shearing interferometer devices located on diametrically opposite sides of the wafer holding mechanism, and a light source optically coupled to the first and second shearing interferometer devices. The first and second shearing interferometer devices are configured to acquire at least two sets of shearing interferograms for each corresponding first and second surfaces of the polished opaque plate. At least one computer coupled to receive the outputs of the first and second shearing interferometer devices is utilized to determine at least one of: a surface slope, a surface curvature, a surface height, a shape, and a thickness variation of the polished opaque plate.
Furthermore, the present disclosure is also directed to a method for measuring the shape as well as its derivatives and thickness variation of a polished opaque plate. The method includes: placing the polished opaque plate within a cavity, the polished opaque plate being held substantially vertically within the cavity utilizing a holding mechanism; simultaneously acquiring two sets of shearing interferograms for each of first and second opposite surfaces of the polished opaque plate; extracting phase maps from the two sets of shearing interferograms for each of first and second opposite surfaces of the polished opaque plate; and calculating at least one of: a surface slope, a surface height, a shape, and a thickness variation of the polished opaque plate based on the extracted phase maps.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the present disclosure. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate subject matter of the disclosure. Together, the descriptions and the drawings serve to explain the principles of the disclosure.
The numerous advantages of the disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which:
Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings.
Silicon wafers are available in a variety of sizes. They may also be patterned, and depending on the specific patterns applied or the lack of such patterns (which are referred to as bare wafers), they may warp in different ways to varying degrees. It has been observed that efficiencies of Fizeau interferometer based wafer shape and thickness measurement systems may need to be improved when wafer warp exceeds certain limit.
The present disclosure is directed to an alternative apparatus and method to Fizeau interferometer based measurement system for rapidly measuring the shape and thickness variation of a wafer. More specifically, shearing interferometry techniques are utilized in measurement systems in accordance with the present disclose. Such measurement systems are able to measure wafer shapes with warp greater than 150 micrometers (μm) without sub-map stitching. Furthermore, shearing interferometry techniques utilized in the manner in accordance with the present disclose also improve the measurement accuracy and precision in noisy environments.
Unlike a normal interferometer, such as Fizeau interferometer that obtains the surface height directly from the interferogram, the information directly obtained from the interferogram of a shearing interferometer is the surface slope. In other words, a shearing interferometer does not need a process of taking derivatives from a height map to get slope maps while a normal interferometer has to do so. Since such process is a high pass filtering process that increases noises, the slope maps accomplished from a shearing interferometer have better signal to noise ratio than that from a normal interferometer. Therefore, in addition to the ability to measure wafers with large warps and being insensitive to vibration, using shearing interferometers also improves measurement of metrics defined on the slope maps and metrics defined on the curvature, such as the wafer stress or the like.
Referring to
As depicted in
More specifically, in accordance with the present disclosure, the measurement system provides two light sources for Channel A and Channel B through fiber 22 and fiber 42 from a single illuminator 8 that generates a constant power output. In one embodiment, the light source 24,44 provides light that passes through a quarter-wave plate 28,48 aligned at 45° to the polarization direction of light after it is reflected from the polarizing beam splitter 26,46. Alternatively, the measurement system may be built by removing the quarter wavelength plate 28,48 and replacing the polarizing beam splitter 26,46 with normal beam splitters. In either implementation, the beam propagates to the lens 30,50, where it is collimated.
This collimated beam is then reflected from the wafer surface 61,62 and travels back to the lens 34,54, where it is collimated again. The collimated beam now reaches the shearing camera 36,56, where the wavefront laterally sheared into two wavefronts.
It is understood that the optical setup of a shearing camera as described above are merely exemplary; various other shearing techniques may be utilized by the shearing camera 36,56 without departing from the spirit and scope of the present disclosure. It is also contemplated that beam splitters can be added to divide a beam into multiple beams so that multiple shearing interferogram can be achieved simultaneously with multiple shearing cameras in different shearing directions.
Now, referring again to
In one embodiment, the three edge grippers 80 are distributed along the circumference of the wafer 60. The reason that three edge grippers 80 are used is because three points define a plane. Using less than three grippers may not be able to hold the wafer 60 steadily in a defined plane, and using more than three grippers, on the other hand, may introduce undesired tension on the wafer 60 if one of the grippers is not perfectly aligned with the others. Three edge grippers that vertically hold the wafer at its edge minimizes wafer distortions and/or shape changes. In addition, the edge grippers do not block light beam to any parts of the measuring surfaces and therefore it is possible to measure both sides of the wafer 60 at the same time. It is understood, however, that the three-gripper configuration as described here is exemplary; more than three edge grippers may be used without departing from the spirit and scope of the present disclosure.
Utilizing the shearing interferometers in conjunction with the vertical holding mechanisms in accordance with the present disclosure provides several advantages over existing measurement systems. For instance, the shearing interferometers are insensitive to vibration and air turbulence, and they are able to measure large wafers and/or wafers with large curved/warped surfaces. In addition, the ability to measure both sides of the wafer allows wafer shapes and thickness variations to be measured, which are not supported using conventional shearing interferometers. It is contemplated, however, that the measurement system may be built with one shearing interferometer with the edge grippers to measure one of wafer surfaces in certain applications.
Referring now to
Step 604 may then acquire four sets of shearing interferograms simultaneously, two for each side of the wafer. Step 606 may extract phase maps from these four sets of shearing interferograms, and step 608 may compute all required maps from these phases extracted in step 606. The desired information includes the slope and the curvature of surface heights, surface heights, wafer shape, and thickness variation of the wafer. All desired metrics can then be computed from these maps.
For instance, if the objective is to obtain metrics based on slope maps on one side of wafer surface only, the process may include: 1) obtain two sets of shearing interferograms with shearing direction in x- and y-directions respectively from the corresponding side of the interferometer system; 2) compute x slope map and y slope map based on the two sets of shearing interferograms; and 3) compute metrics from the slope maps. In another example, if the objective is to obtain metrics based on curvature maps on one side of wafer surface only, the process may include: 1) obtain two sets of shearing interferograms with shearing direction in x- and y-directions respectively from the corresponding side of the interferometer system; 2) compute x slope map and y slope map based on the two sets of shearing interferograms; 3) compute x curvature map from x slope map and y curvature map from y slope map; and 4) compute metrics from the curvature maps. It is contemplated that these processes may be extended to both sides of the interferometer system if the objective is to obtain metrics from the slope maps and/or curvature maps on both sides of wafer surfaces.
In another example, if the objective is to obtain wafer shape information, the process may include: 1) obtain two sets of shearing interferograms with shearing direction in x- and y-directions respectively from the corresponding side of the interferometer system; 2) compute x slope map and y slope map; 3) integrate with x slope and y slope to get the surface shape; and 4) repeat these steps to get surface shape on the other side of wafer. The wafer shape information can then be determined from one of the surface shapes or from both surface shapes. In one embodiment, if both sides of the wafer surface are utilized, the wafer shape information can be calculated as
and metrics can be calculated based on this wafer shape information.
In still another example, if the objective is to obtain wafer thickness variation, the process may include: 1) obtain four sets of shearing interferograms, two for each side with shearing direction in x and y-directions respectively; 2) compute x slope map and y slope map for the front side of wafer and compute x slope map and y slope map for the back side of wafer as well; 3) integrate with x slope and y slope from the same side to get the surface shape for both the front and the back of wafer surfaces; and 4) the wafer thickness variation can be calculated as the difference between the front surface shape and the back surface shape. In one embodiment, the wafer thickness variation=the front surface shape−the back surface shape, and metrics can be calculated based on this wafer thickness variation.
It is contemplated that the calculation processes described above are merely exemplary. Other types of metrics and calculation processes may be utilized and/or implemented without departing from the spirit and scope of the present disclosure. It is also contemplated that steps in method 600 may be carried out multiple times in order to increase the precision and accuracy of the measurement result. The number of iterations to be performed may be customized to meet requirements demanded by different users and/or for different types of wafers.
It is to be understood that the present disclosure may be implemented in forms of a software/firmware package. Such a package may be a computer program product which employs a computer-readable storage medium/device including stored computer code which is used to program a computer to perform the disclosed function and process of the present disclosure. The computer-readable medium may include, but is not limited to, any type of conventional floppy disk, optical disk, CD-ROM, magnetic disk, hard disk drive, magneto-optical disk, ROM, RAM, EPROM, EEPROM, magnetic or optical card, or any other suitable media for storing electronic instructions.
The methods disclosed may be implemented as sets of instructions, through a single production device, and/or through multiple production devices. Further, it is understood that the specific order or hierarchy of steps in the methods disclosed are examples of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the method can be rearranged while remaining within the scope and spirit of the disclosure. The accompanying method claims present elements of the various steps in a sample order, and are not necessarily meant to be limited to the specific order or hierarchy presented.
It is believed that the system and method of the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described is merely explanatory.
The present application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/807,090, filed Apr. 1, 2013. Said U.S. Provisional Application Ser. No. 61/807,090 is hereby incorporated by reference in its entirety.
Number | Date | Country | |
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61807090 | Apr 2013 | US |