Abstract:
A solid immersion lens optics assembly, a test station for probing and testing of integrated circuits on a semiconductor wafer, and a method of landing a SIL on an object. The optics assembly comprises an objective lens housing for receiving an objective lens, and a solid immersion lens (SIL) housing for mounting an SIL and adapted for connection to the objective lens housing; wherein a peripheral wall of the SIL housing comprises an integrated spring section adapted to provide a biased support for the SIL.

Description:
FIELD OF INVENTION 
     The present invention relates broadly to a solid immersion lens (SIL) optics assembly, to a test station for probing and testing integrated circuit on a semiconductor wafer and to a method of landing a SIL on an object. 
     BACKGROUND 
     Microscopes are used in a large variety of technological applications, including in probing and testing of semiconductor microchips. With increasing metal layers and flip chip bonding, analysis of the integrated circuit (IC) can typically only be done from the backside of the chip through the silicon substrate using infrared imaging. Shrinking device geometry requires high numerical aperture (NA) lenses to resolve the transistors. In such applications, the optics plays a crucial part in device imaging, signal collection and optical probing, and in particular when the signal is optically weak. 
     In one existing type of optics for, e.g., probing and testing of semiconductor microchips, a SIL is placed between the object and an objective lens, with or without the use of an index matching medium between the SIL and the semiconductor microchip. The increased NA of the SIL-Objective arrangement allows higher resolution imaging, higher signal collection efficiency and smaller spot size for optical probing. 
     In such SIL-Objective optical arrangements, the challenges are to maintain the SIL and objective optical axis alignment when placing and focusing the SIL on the device, accommodating a small degree of tilt between device and SIL, and applying bias to eliminate the air gap at the SIL-device interface. 
     One existing solution in U.S. Pat. No. 7,123,035 provides the SIL attached to a bracket that is spring loaded by springs to a housing containing the objective lens. The springs extend readily inwardly from the housing and are coupled to the bracket carrying the SIL, substantially at a periphery of the SIL. Specific details as to the nature of the springs and the actual connection of the springs to the housing on the one hand, and the bracket carrying the SIL on the other hand are not provided. However, it is believed that in such a design there would be a number of practical implementation issues such as a potential vignetting effect from the springs, in particular in the biased stage upon landing of the SIL, as well as issues relating to choice of the number of springs to be used, and/or uniformity of the applied bias. 
     In another design described in U.S. Pat. No. 7,123,035, the SIL is fixedly attached to a SIL housing, and the objective lens is fixedly attached to an objective housing. Both the SIL housing and the objective housing are substantially cylindrical, with the SIL housing being received within the objective housing, and in a manner such that a sliding motion of the SIL housing relative to the objective housing is enabled in a biased fashion. One or more linear springs are disposed within the objective housing at its periphery, for biasing the SIL housing. Disadvantages or challenges associated with such a design include the limited, if any, ability of accommodating angular displacement between the SIL housing and the objective housing, for example as a result of landing on a surface slanted with respect to the SIL and objective housings, as well as friction between the SIL housing and the objective housing, resulting in backlash during focusing. Furthermore, since a sliding fit between the SIL housing and objective housing is required, as there is relative movement, the centering and imaging repeatability is limited by the tolerances required for the sliding fit. 
     A need therefore exists to provide an alternative system and method that seek to address at least one of the above-mentioned problems. 
     SUMMARY 
     In accordance with a first aspect of the present invention, there is provided a solid immersion lens (SIL) optics assembly comprising an objective lens housing for receiving an objective lens, and a SIL housing for mounting a SIL and adapted for connection to the objective lens housing; wherein a peripheral wall of the SIL housing comprises an integrated spring section adapted to provide a biased support for the SIL. 
     The wall of the SIL housing may be cylindrical, and the spring section may comprise a radial spring along a circumference of the cylindrical housing. 
     The radial spring may comprise a single start helix or multiple start helix spring. 
     The assembly may further comprise a SIL holder adapted for mounting the SIL, and adapted for connection at a top end of the SIL housing. 
     The SIL holder may comprise a conical mount element extending from a base portion, the conical mount element adapted for mounting the SIL at an apex thereof. 
     The assembly may further comprise a spacer element for interconnection between the SIL housing and the objective lens housing. 
     Components of the assembly are adapted for threaded engagement with each other. 
     The assembly may further comprise the objective lens. 
     The assembly may further comprise the SIL. 
     The objective lens housing may be adapted for adjustment of a position of the received objective lens. 
     In accordance with a second aspect of the present invention there is provided a test station for probing and testing of integrated circuits on a semiconductor wafer, the test station comprising a SIL optics assembly as defined in the first aspect. 
     In accordance with a third aspect of the present invention there is provided a method of landing a SIL on an object, the method comprising the steps of coupling an objective lens housing having an objective lens received therein to a SIL housing having mounted thereon a SIL; and providing a biased support for the SIL using a spring section integrated in a peripheral wall of the SIL housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which: 
         FIG. 1  shows a design drawing showing an exploded side view of a SIL optics assembly according to an example embodiment; 
         FIG. 2  shows a design drawing showing an exploded perspective view of the SIL optics assembly of  FIG. 1 ; 
         FIG. 3  shows a schematic drawing illustrating a test station for probing and testing integrated circuits on a semiconductor wafer according to an example embodiment; 
         FIG. 4  shows a flowchart illustrating the initial setup and positioning of the SIL optics assembly according to an example embodiment; and 
         FIG. 5  shows a flowchart illustrating a method of landing SIL on an object, according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a design drawing showing an exploded side view of a SIL optics assembly  100  according to an example embodiment. The SIL optics assembly comprises an objective lens  102 , received within a lens adaptor  104 . The lens adapter  104  is adapted for threaded engagement with a spring tube  108 . A spacer ring  106  is provided for the SIL optics assembly length adjustment. It will be appreciated that the spacer ring  106  is optional and can be of different height, depending on a desired configuration of the SIL optics assembly  100  for different SIL diameters, and centric and aplanatic imaging applications. 
     A SIL holder  110  is provided for threaded engagement with the spring tube  108 . The SIL holder  110  comprises a raised conical portion  112  for attachment of a solid immersion lens (SIL)  114 . The SIL  114  is attached to the SIL holder  110  using epoxy in this example embodiment, but it will be appreciated that other attachment means may be used in different embodiments. 
     In the example embodiment, relative movement required during focusing of the SIL holder  110  with respect to the objective lens  102  is provided by way of the spring tube  108 , which includes an integrated radial spring section  116  forming a hollow cylindrical support for the SIL holder  110  in the example embodiment. The adjustment set screws  111  on the lens adapter  104  allow pre-alignment of the SIL  114  optical axis to the objective optical axis. The hollow cylindrical support preferably maintains the alignment during relative axial movement when focusing. 
     The compression of the spring tube  108  at and near focus provides a force to eliminate the minute air gap between the SIL  114  and a device under test (DUT) surface, reducing reflection losses and total internal reflection at the interface, thus preferably avoiding NA reduction. It will be appreciated that the spring tube  108  advantageously provides a biased support for the SIL holder  110  in a manner such that shadowing of the SIL  114  by the biasing member can be eliminated. 
     It will be appreciated that the lens adapter  104  allows exchange of objective lenses  102  of different magnifications and NA to match the type of SIL  114  used. Spring tubes  108  of different spring stiffness can also be used. Increasing the stiffness and thus axial stability of the spring can advantageously improve repeatable SIL placement for imaging and probing. Also, while the radial spring can be formed as a single start helix spring as described in the example embodiment, double or more start helix springs can be implemented in different embodiments to increase axial stability for improving repeatable SIL placement, and to reduce side loading. 
     The SIL holder  110  can be custom made to hold SILs  114  of different diameters and thickness to cater for DUTs of different substrate thickness. Spacer rings  106  of different heights can be used to adjust the initial offset of the SIL  114  from the par focus of the lens assembly; and thus the compression and force of the spring tube  108  at focus. Furthermore, the spring tube  108  can preferably accommodate a small angular displacement of the SIL holder  110  relative to a central axis  118  of the SIL optics assembly  100  without or with a reduced risk of the SIL holder  110  being “stuck” relative to the objective lens  102 . Alternatively or additionally, the DUT may be levelled during operation to achieve better image quality. 
     In the example embodiment, the lens adapter  104 , spacer ring  106 , spring tube  108  are fabricated from aluminium. The SIL holder  110  is fabricated from stainless steel. The SIL holder can be made from copper or brass if thermal dissipation through the SIL  114  is required. The spring tube  108  can be made from stainless steel to vary the spring rate and thermal conductivity depending on application. 
     The SIL  114  material can be gallium arsenide (GaAs) or silicon (Si) in example embodiments. The choice of material typically depends on the wavelength. For example, GaAs is typically used for 1000-1200 nm wavelength and Si for 1200 nm onwards. The SIL  114  can be hemispheres, used for imaging at the centric point or hyper-hemispheres, used for imaging at the aplanatic point. The plano surface of the SIL  114  can also been made convex for better conformance to the device surface. 
     The lens adapter  104 , spacer ring  106  and spring tube  108  are fabricated by turning, and the SIL holder  110  is fabricated by turning and CNC in the example embodiment. 
     It will be appreciated by a person skilled in the art that other suitable materials and other fabrication techniques may be used in different embodiments for the respective parts without departing from the spirit or scope of the invention. 
       FIG. 2  shows a design drawing showing an exploded perspective view of the SIL optics assembly  100  of  FIG. 1 . In  FIG. 2 , the same reference numerals are used to identify the same parts compared to  FIG. 1 . 
       FIG. 3  shows a schematic drawing of a test station  300  for testing and optical probing of packaged flip chip integrated circuit or device under test (DUT)  301 . As will be appreciated by a person skilled in the art, the silicon substrate of the DUT  301  is first thinned and electrical functionality validated. A typical test involves inserting the DUT  301  into a test socket  303  on a load board  305  whilst using a SIL optics assembly  307  to conduct imaging and optical probing through the silicon substrate of the DUT  301 . 
     The test socket  303  and the load board  305  constitute the interface between an electronic test system  310  and the DUT  301 . The load board  305  is docked to the microscope system and the docking interface can be fitted with tip and tilt adjustments  320   a  and  320   b  for levelling the DUT  301  normal to the SIL optics assembly  307 . 
     The SIL optics assembly  307  is part of a microscope  309  mounted on a scope transport  311  for x, y, z manipulation relative to the backside of the DUT  301 . The scope transport  311  in the example embodiment is adapted for manual as well as computer controlled movement. 
     In the initial set up of the SIL optics assembly  307 , the SIL  313  is set at a distance (e.g. 2 mm) further than the par focus of the backing objective lens  315  using the spacer ring (compare  106   FIG. 1 ). The SIL  313  is centered to the backing objective lens  315  using the alignment set screws (compare  111   FIG. 1 ). In operation, lower magnification objective lenses  317  on the microscope  309  are used to navigate to the point of interest on the DUT  301 . The SIL optics assembly  307  is selected and the microscope transport  311  adjusted to the par center. The microscope transport  311  then lowers the SIL optics assembly down to the par focus position. At the par focus position, the spring tube  319  compression is based on the initial distance offset from the par focus. The spring constant and the compression distance determine the force pressing the SIL  313  and DUT  301  surface together. The microscope transport  311  can be used to adjust the distance between the objective  315  and SIL  313  for fine focus. 
     As the SIL  313  and DUT  301  are in contact, precautions are preferably observed to prevent damage. Prior to landing, the x, y lateral movement of the microscope transport  311  is disabled after par centre adjustment. This advantageously prevents scratches to the SIL  313  and DUT  301  if accidental x, y movement of the microscope transport  311  occurs. Gross z movement is also disabled to prevent crushing of the SIL  313  or DUT  301 . Fine z movement is permitted for focusing, with movement range about the par focus point restricted by software limits in the example embodiment. 
     In this example embodiment, the test station  300  does not have an image/video analyser. The image of the DUT or image of photon emissions are displayed on the Personal Computer (PC)  322  screen coupled to the microscope  309  for “manual” visual inspection. However, it will be appreciated that an image/video analyser can be provided in different embodiments. 
       FIG. 4  shows a detailed flowchart  400  illustrating the initial setup and positioning of the SIL optics assembly onto a DUT according to an example embodiment. At step  402 , the spacer ring is used to offset the SIL from par focal point. The amount of offset determines the contact force when the lens is at par focus (at step  412  below). 
     Next, at step  404 , the SIL is aligned to the objective optical axis using adjustment screws on the lens adaptor in the example embodiment. Steps  402  and  404  are performed as a one time setup in the example embodiment. 
     Next, a tilt of the DUT is adjusted at step  406 , and navigation to the point of interest is performed using the microscope transport and lower magnification objective in place of the SIL optics assembly including the SIL in step  408 . After the initial navigation to the point of interest in step  408 , the SIL optics assembly including the SIL is selected instead of the low magnification objectives at step  410 . 
     Next, under computer control, the x, y, z, course controls are disabled, and the SIL par center offset is corrected by the microscope transport. Under a preset computer control, the SIL is lowered to par focal point, thus landing the SIL on the DUT at step  412 . 
     Next, at step  414 , manual fine z movement is enabled to adjust fine focus. If it is determined at step  416  that the location of the SIL is correct, the testing is performed at step  418 . In an example embodiment, the correct location is determined visually or with the aid of computer aided design (CAD) navigation software. There may be two scenarios, for example. First, if the signal from the DUT (either photon emission or laser induced) has been picked up with the lower magnification objectives, the operator uses the SIL lens to obtain a higher magnification and resolution image to more accurately localise the defect. CAD navigation is used to determine the co-ordinates of the location. The second scenario can be that the operator wishes to use a laser beam to probe one or a group of transistors. The operator will use CAD navigation software to move to and identify the correct location. For both scenarios, the operator determines the location of interest. If at step  416  the location is found to be incorrect, at step  420  the microscope transport is raised using the preset computer control, to separate the SIL from the DUT. X, y, movement of the microscope transport is performed using the preset computer control, and the microscope transport lowered to land the SIL to the last focal point position. Thereafter, steps  414  and  416  are repeated until a correct location is confirmed and the testing can be performed at step  418 . 
     At step  422 , the operator initially determines the new location but is not allowed direct control of the stage movement to avoid damage from a wrong sequence of moves. Usually the location is within the image field of view and the operator double clicks on the location on screen for the software to calculate the relative x, y movement required to bring the location to the center of the screen. If it is not in the field of view, the operator will have to decide the direction of panning and the software will move x, y by one field of view. 
       FIG. 5  shows a flowchart  500  illustrating a method of landing a SIL on an object according to an example embodiment. At step  502 , an objective lens housing having an objective lens received therein is coupled to a SIL housing having mounted thereon a SIL. At step  504 , a bias support for the SIL is provided using a spring section of a peripheral wall of the SIL housing. 
     The optics assembly of embodiments of the present invention can be implemented e.g. for photon emission microscopy. In either device imaging or emission imaging, the optics assembly can be used as ‘collection’ system. In a scanning optical microscopy implementation, for device imaging, the optics assembly can be used as ‘collection’ system, while in laser induced imaging where the DUT response is imaged, the optics assembly can be used as a ‘probe’. 
     It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.