Abstract:
A system and method for correction of aberrations in a solid immersion microscopy system using a deformable mirror. A solid immersion lens is provided having a surface configured to make optical contact with a nearly planar surface of a substrate, an object to be imaged disposed on the opposite side of the substrate. A convex surface of the solid immersion lens faces an objective lens. A deformable mirror assembly, including a plurality of individually controllable actuators, receives light transmitted from the object. A control system controls in communication with the deformable mirror assembly provides individual actuation of each of the actuators of the deformable mirror to compensate or counteract the effects of aberrations.

Description:
[0001]    STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
         [0002]    This invention was made with Government Support under Contract No. FA8650-11-C-7102 awarded by the Air Force Research Lab. The Government has certain rights in the invention. 
     
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0003]    N/A 
       BACKGROUND OF THE INVENTION 
       [0004]    Solid immersion lenses (SILs) are used in semiconductor failure analysis to increase the resolution and signal collection of optical microscopes for backside photon-based imaging and failure analysis techniques. The improvement in resolution allows optical techniques to be extended to the most modern devices, where the features are sufficiently small that they cannot be imaged without an immersion lens. These lenses are currently incorporated into commercial failure analysis equipment. 
         [0005]    When imaging with an immersion lens many factors lead to aberrations, such as off-axis imaging, errors in fabrication of the surfaces, and an unknown thickness of the substrate and distance from back surfaces to transistor or other device level. Since spherical aberration free imaging is only possible if the region of interest sits at the center or aplanatic point of the SIL, any mismatch between the design thickness or other variable and the actual thickness or other variable of the substrate or other element in the light path will rapidly cause an increase in the focus spot size. 
       SUMMARY OF THE INVENTION 
       [0006]    A microscopy system for aberration correction is provided, particularly for the application of commercial semiconductor failure analysis and semiconductor fabrication quality control. The microscopy system incorporates a solid immersion lens for imaging an object, such as a semiconductor, and a deformable mirror (DM) to compensate for aberrations. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1A  is an illustration of a central solid immersion lens (cSIL) configuration, where light rays follow the radii to focus at the center of the lens; 
           [0009]      FIG. 1B  is an illustration of an aplanatic solid immersion lens (aSIL) configuration, where light rays refract at the curved surface, focusing to the aplanatic point, a distance R/n below the center of the immersion lens, where R is the radius and n the index; the aSIL has a larger numerical aperture (NA) allowing potentially higher resolution; 
           [0010]      FIG. 2  is a schematic diagram of one embodiment of a solid immersion microscopy system with a deformable mirror assembly; many alternate configurations are possible, which would in general also include a beam scanner system as known in the art. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Solid immersion lenses (SILs) are generally of a plano-convex design, with the planar surface sitting in optical contact with the substrate of the device under test (DUI) and the convex surface facing the backing objective in the imaging system. Variations of this basic design include shaped surfaces (for example, slightly convex, toroidal, etc.) rather than a planar surface for the SIL surface that contacts the substrate. Two types of SIL geometry are illustrated in  FIGS. 1A and 1B . Both of these types allow the possibility to perform aberration-free imaging. The first type, in  FIG. 1A , known as a central SIL (cSIL), images light at the center of the radius R of curvature of the immersion lens  10 . That is, the light rays follow the radii to focus at the center of the lens. The second type, in  FIG. 1B , known as an aplanatic SIL, images light at the aplanatic point  12  of the immersion lens  14 , a distance R/n below the center  16  into the substrate  18 , where n is the refractive index. That is, light rays refract at the curved surface, focusing to the aplanatic point, a distance R/n below the center of the immersion lends. In a perfect, on-axis, model system, both designs are aberration free. The aplanatic SIL (aSIL) also has a larger numerical aperture (NA), allowing potentially higher resolution. This aSIL is often referred to as a numerical aperture increasing lens, or NAIL. 
         [0012]    A SIL is used generally to increase the microscope NA, which in turn yields better potential resolution. And while the cSIL and aSIL can theoretically produce aberration free images at one point in the DUT, they are extremely susceptible to optical performance degradation due to errors in the SIL geometry or due to imaging at points on the DUT other than the optimal point, since the impact of aberrations scales with the optical system&#39;s numerical aperture, NA. Since the general purpose of microscopy is to image over a field-of-view that exceeds one point, the effect of aberrations in high-NA imaging can be significant, and can degrade image quality severely. 
         [0013]    When imaging a DUT with an immersion lens of either central or aplanatic type, many factors can lead to aberrations: off-axis imaging, errors in fabrication of the surfaces, and an unknown thickness of the substrate and distance from back surface to the object plane to be imaged. Since aberration-free imaging is only possible if the region of interest sits at the center or aplanatic point of the SIL, any mismatch between the thickness for which the SIL is designed and the actual thickness of the substrate causes an increase in the aberrations present and hence increases the focused spot size. 
         [0014]    On axis aberrations include defocus and spherical aberrations. Off-axis aberrations include astigmatism, coma, trefoil, and other higher order aberrations. Aberrations can also arise from differences between the refractive index of the substrate of the DUT and the refractive index of the SIL. Aberrations can be introduced by imperfections in the silicon substrate, for example, densification and rarification of the substrate when polished. Errors in the alignment or positioning of other optical components in the microscope can introduce aberrations. Other aberrations due to imperfections in the SIL manufacturing process, such as asphericity, can arise. Those of skill in the art will recognize that aberrations are not limited to these just discussed. 
         [0015]    Another source of aberrations is the shape of the SIL itself. In the cSIL geometry, there is little or no refraction on the steeply sloped convex surface, making that system less susceptible to optical errors due to shape errors. However, in aSIL geometry, substantial refraction at the convex surface makes that system more susceptible to wavefront distortion due to aSIL shape errors. 
         [0016]    As an example, the utility of a solid immersion lens (SIL) in semiconductor failure analysis can be hindered if the device under test (DUT) is not prepared to a sufficient accuracy. In particular, the area of interest inside a flip-chip packaged integrated circuit is typically buried beneath 100 μm or more of silicon substrate. Each immersion lens is designed to operate with a specific substrate thickness. If the device is not prepared in such a way that the substrate thickness matches the design thickness of the lens, then the best resolution will not be achieved. Accordingly, a deformable mirror is used in the microscopy system to compensate for this mismatch between the design thickness and the actual thickness. By reshaping the illumination light beam used to image the device it is possible to correct for this mismatch and recover most, if not all, of the resolution. 
         [0017]    Referring to  FIG. 2 , one embodiment of a microscopy system includes an objective lens assembly  20  and a plano-convex aplanatic solid immersion lens (aSIL)  22 . The planar side  24  ( FIG. 1B ) of the aSIL is placed on an object  18 , the device under test, such as a silicon chip, which is supported against a mirror or other reflective backing  26 . 
         [0018]    In the configuration shown, a collimated coherent laser illumination beam  30  from an illumination source  32  via a beam splitter  36  is incident on a deformable mirror (DM)  38 . The plane of the DM is reimaged with a lens pair  40  and  42  (L 1  and L 2 ) onto the pupil plane of the objective lens assembly  20 . It is possible to position the DM elsewhere, but positioning it in a plane conjugate to the objective lens pupil is generally preferred in adaptive optics systems because it allows the most straightforward implementation of control. The objective lens  20  focuses the beam  44  from lens  42  and beam splitter  46  through the aSIL  22  and the backside of the silicon substrate  18  onto the plane of the semiconductor structures that are to be imaged therein. The focused illumination spot  12  reflects back through the objective lens  20  and is partially reflected by the beam splitter  46  (BS 1 ) through a lens  50  (L 3 ) and beam splitter  52  forming an image on a photonic detector  56 , such as a CCD camera or photomultiplier or other photosensitive device. Part of the beam is redirected by a beam splitter  52  (BS 2 ) and another lens  54  (L 4 ) to a wavefront sensor  58  (WFS) that is conjugate to both the plane of the DM and the pupil of the objective lens  20 . In this configuration the WFS  58  can measure any wavefront distortion in the system due to aberrations, and the DM  38  can pre-compensate those distortions either with sensor feedback (i.e. closed loop control) or without it (open loop control). In the former case, the feedback can be provided by the wavefront sensor  58  or by another detector  60  in the image plane that measures resolution, contrast, or some other indicator of the quality of the image or the scanned focal spot. The corrections are made on each of the controllable elements of the DM using information from corresponding areas in the wavefront sensor  58 . In the latter case, the DM control can be based on some known information about the aberrations, such as the presence of first-order spherical aberration due to substrate thickness errors, or the presence of astigmatism due to off-axis imaging. Again, control is on an element-by-element basis. Typically the number of elements can be any desired. Actual experimental DMs have had 140 controllable elements. The SIL and the other lenses in the system can be fabricated from any suitable optical material. 
         [0019]    Any suitable illumination source  32  can be used, generally as determined by the application. The illumination is typically in the visible or infrared range. The illumination source may be a laser coupled via an optical fiber to a collimating lens, or as shown in  FIG. 2 , a source such as a laser of collimated light. In some cases, illumination would not be required and the image could be formed from light emitted by the object itself. For those cases, the DM  38  would be positioned downstream of the objective lens pupil, typically in a conjugate plane with that pupil and the WFS  58 . 
         [0020]    The DM  38  can have a segmented or continuous mirror surface. In one embodiment, the deformable mirror assembly is a continuous face-sheet microelectromechanical system (MEMS) having an array of deformable mirror elements or actuators. The mirror elements are individually actuatable by electrostatic forces. Suitable MEMS deformable mirrors are commercially available from Boston Micromachines Corporation. The various parameters, such as pitch or center-to-center distance between actuators, stroke, number of actuators, response time, and size of the array, can be selected based on the application. 
         [0021]    In one embodiment, suitable for imaging a silicon chip of 100 μm thickness, a DM  38  with 140 electrostatic actuators is used. The deformable mirror includes actuators capable of 5.5 μm stroke, oriented in a square array on 400 μm pitch between actuators. Other DMs, including those with piezoelectric, magnetic, and thermal actuation schemes can be used, and more or fewer actuators on various pitches are possible. For this application the dominant aberrations are likely to be spherical aberration, astigmatism, coma, and trefoil (i.e., relatively low order optical aberrations). In another embodiment, a DM can be specifically designed to compensate anticipated aberrations, with actuators oriented in circular or other geometries to better match the known effects of such aberrations on wavefront distortion. For example, a DM with several ring-shaped actuators could efficiently control spherical aberrations with fewer degrees of freedom than a DM with actuators oriented in a square grid. 
         [0022]    In one embodiment, to control wavefront distortion, the deformable mirror  38  is controlled in an open-loop fashion, using techniques developed previously by Diouf et al. (Diouf, A., LeGendre, A. P., Stewart, J. B., Bifano, T. G., Lu, Y., “Open-loop shape control for continuous microelectromechanical system deformable mirror.”  Applied Optics , Vol. 49, No. 31 (2010), pp 148-154.) (Appendix A). With this control approach, arbitrary shapes can be made on the DM, with shape errors typically less than 25 nm rms. This approach could be used, for example, to control aberrations due to substrate thickness errors. Such errors are common, as it is often difficult to produce a substrate of precisely the desired thickness. Moreover, it is sometimes required that images be made of several different layers in a substrate. There is only one plane at which a SIL-based image will be aberration free. Such substrate thickness errors will result in spherical aberration, and the effect of this aberration on wavefront is well-known. Thus, if the substrate thickness errors can be measured, the DM can be shaped appropriately to counteract expected wavefront distortion without any real-time feedback. 
         [0023]    In another embodiment, to control wavefront distortion, the deformable mirror is controlled in an closed-loop fashion. Known adaptive optics techniques can suitably be used. As one with familiarity in adaptive optics would know, the standard closed loop approach is to build an empirical reconstructor matrix to map wavefront sensor measurements to DM shapes in a pre-control calibration step, then iteratively measure WFS as input, and multiply the WFS input by the inverted reconstructor matrix to get an estimate of shape errors at the DM, then use integral control to update the DM outputs accordingly. With this control approach, shapes made on the DM  38  are prescribed using feedback from a wavefront sensor or an image quality metric or some other sensor indicative of the relative state of uncompensated aberrations in the optical system. An iterative controller  62  updates the DM repeatedly in an effort to optimize either the wavefront flatness or the image quality. Through this technique, wavefront errors can typically be reduced to a small fraction of the optical wavelength in amplitude. 
         [0024]    A combined approach is also possible, in which open loop and closed loop control techniques are applied is series or in parallel. 
         [0025]    With the present aberration correction system, semiconductor inspection systems and failure analysis systems can operate with chips of significantly higher complexity than is possible with prior art systems. Also, the present aberration correction system extends the abilities of imaging systems to resolve smaller feature sizes. 
         [0026]    It will be appreciated that many other configurations of the optical system can be employed besides that shown in  FIG. 2 . The system also includes a beam scanner assembly, which for simplicity is not shown in  FIG. 2 . The system can also be used for other SIL microscopy applications in which it is required to image through a substrate many times thicker than the wavelength of the light used to generate an image, where the imaged features have dimensions measuring only a small fraction of the wavelength. Such applications are not restricted to semiconductor fabrication, but might also include biological imaging applications and subsurface imaging with optical wavelengths from millimeter to nanometer scales. 
         [0027]    It will be appreciated that features of the various embodiments and examples described herein can be combined in different ways from those explicitly shown and described. The invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.