Patent Abstract:
A method and apparatus for holding and delayering a die include an outer member ( 10 ) that receives an inner member ( 20 ), and a set screw ( 25 ) and set screw hole ( 12 ) for securing the position of the inner member ( 20 ) within the outer member ( 10 ). A die ( 50 ) is attached to the inner member ( 20 ), and the apparatus is then used to apply the die ( 50 ) to an abrasive disk ( 200 ) which is attached to a rotatable wheel ( 300 ) and is delayered by progressive abrading. The outer member ( 10 ) provides stability and precision to the delayering operation. The inner member ( 20 ) provides portability and control to the delayering operation.

Full Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to methods and tools used in failure analysis of integrated circuit (IC) products in the semiconductor industry, and more particularly to a method of mechanically delayering a semiconductor die (also called an integrated circuit chip) in a controlled manner, and an apparatus for carrying out the method. 
     IC circuits fail due to various physical, chemical or mechanical mechanisms such as electrical overstress, contamination, or wear out. Some failure analysis approaches and procedures require a die to be precisely delayered down to a particular layer to locate such mechanisms. The most well known method of mechanically delayering a die involves progressively abrading the die using a die holder, an abrasive, and a rotatable wheel. The die holder applies the die to the abrasive that is attached to the rotatable wheel. 
     The die holders currently used are often problematic and limited in their usefulness. These problems and limitations result from instability, imprecision and lack of portability. The prior art has attempted to address these concerns but has fallen short of producing desired and reliable results. 
     Instability, imprecision and lack of portability adversely affect delayering analysis in several ways. The conventional and most popular method is to secure a die to a die holder, then manually apply the die holder to a rotatable wheel, with the die exposed and sandwiched in-between. The disadvantage of this method is that it introduces inconsistent conditions due to finger pressure variance. Finger pressure variance causes certain portions of the die to be delayered at a faster rate, resulting in non-uniform abrading across the die. Finger pressure variance also significantly changes the abrading angle between the to-be-abraded die surface and the rotatable wheel. This lack of control of the force and directional components results in undesired die surface characteristics, which can be detrimental to delayering analysis. As will be discussed in detail later, delayering the die produces some die surface characteristics that are desirable and some that are undesirable. Lack of control in the delayering process is problematic when the failure mechanism evidence is destroyed from too much delayering. 
     Another method is to use a delayering attachment with a polishing machine. This method is intended to eliminate finger pressure variance, but instability of the die holder has been known to crack the die or, too often, delayer only one corner of the die. Users continue to use the attachment but therefore often revert to using it with the manual method as described above, rather than with the machine. This reintroduces the finger pressure variance problem. 
     Lack of portability also contributes to user problems. Lack of portability is the inability of the die holder to be directly used with other failure analysis equipment, for example, an optical microscope, a scanning electron microscope, or a plasma or dry etcher. Thus, prior art devices require the user to detach the delayered die from the holder, and then inspect the die in the appropriate analysis equipment, with another type of holder or without any holder. When more delayering is needed, the user places the die back onto the die holder for more delayering. This introduces undesired variables in the die position, so that if the die is tilted differently or rotated from its position when previously delayered, the abrading produces undesired die surface characteristics, as will be discussed in detail later. The analysis for that particular die is then at an end. 
     A need therefore remains for a mechanical die delayering method and apparatus that precisely control the abrading angle such that the die is abraded more uniformly. The apparatus also needs to be portable allowing the user to place the die sample in other failure analysis tools without having to remove the die from the die holder 
     Accordingly, the first objective of the invention is to control the abrading angle. The abrading angle is the angle between the die surface to be abraded and the rotatable wheel. When the die is abraded using a rotatable wheel, a rainbow appears on the die. The rainbow rings on the die can adversely affect visual analysis of particular die circuits, specifically when the rings pass over and obscure transistors of interest. There is no known method of eliminating these rings. The rainbow effect, however, is not a problem when there is a sufficient distance between rainbow rings and the direction of the rings can be controlled. While the required distance will vary with the size of the circuit to be analyzed, a distance of 10 microns will usually suffice; a distance of 1000 microns is ideal. To obtain the maximum distance between rainbow rings, the abrading angle must be decreased to and sustained at a maximum of one degree. 
     The second objective is to abrade the die more uniformly by decreasing any wobbling that might occur during the delayering procedure. Any wobbling increases the pressure differential. This causes multidirectional rainbow rings, too many of which impair visual analysis of the die. Uniform delayering produces desired unidirectional rainbow rings. 
     The third objective is to allow the user to place the die sample in other failure analysis tools without having to remove the die from the die holder. The present invention provides that capability by enabling the die to be used intermittently in different tools during the entire delayering process without ever changing its position relative to the die holder. 
     SUMMARY OF THE INVENTION 
     The present invention meets the above needs and objectives with a new and improved method of delayering a die in a controlled manner, and a new and improved die holder therefor in which the geometry is such that it provides stability and precision to a die delayering process. 
     In the preferred embodiment, the die holder according to the present invention includes concentric inner and outer cylinders which can be axially adjusted relative to each other. The die which is to be delayered is attached to one end of the inner cylinder, and this end of the cylinder, with the die attached, is positioned just inside the corresponding surface of the outer cylinder so that the die is barely exposed above the adjacent outer cylinder surface. The cylinders and die are then locked in this position, such as by a set screw, following which the assembly is applied against a conventional rotating wheel and abrasive to delayer the die as desired. 
     The present invention provides precise control, stability, and precision to the process through the ability to carefully control the amount that the die is exposed (the “exposure increment”) and the maximum possible angle (“wheel angle”) at which the die surface may be removed or abraded. The dimensions of the cylinders (and particularly the width of the outer cylinder surface) are maintained to keep the wheel angle, ideally, less than one degree. The exact dimensions, of course, will depend upon the actual size of the die, transistor geometries, and the anticipated variations from one sample to another, including variations in the adhesive thickness which holds the die on the end of the inner cylinder. The end result produces the desired delayering or die surface characteristics (the so—called broad “rainbow rings”) on a consistent, reliable, and easily produced basis. 
     The adjustability of the cylinders relative to each other, so that only a very small exposure increment needs to be utilized, also provides for keeping the die holder quite compact relative to die analysis equipment, yet with no loss of precision and performance in the delayering operation. This means that the die can remain attached to the mount during subsequent analysis operations, and then for still further delayering operations thereafter. Not only is this significantly more convenient for the user, but it expedites the diagnostic processes and provides for substantially improved precision in such sequential operations. 
     It is therefore an object of the present invention to provide a new and improved method and apparatus for delayering dies; wherein the die holder comprises, in combination, an inner member, one surface of the inner member being at least as large as the die which is to be delayered, the outer member having an exteriorly open hole therein, the hole being large enough to removably receive the inner member entirely therein, the hole also being deep enough to provide for receiving the inner member in a position wherein the one surface of the inner member would be beneath the exterior of the hole at a depth substantially as deep as the thickness of the die which is to be delayered, the members being configured to permit the inner member to move inwardly and outwardly through the hole opening, at least one positioner connected to at least one of the members for positioning the inner member relative to the opening, such that, when the die is attached to the one surface, the exposed to-be-delayered die surface is exposed at a predetermined increment just outside the hole opening, the predetermined increment subtending an exposure angle between the highest plane defined by the surface of the outer member around the hole opening and the highest point of the exposed to-be-abraded die surface above the plane, and the predetermined increment defining the exposure angle to be less than substantially two degrees; in which a lock may be configured for locking the inner member relative to the outer member, the lock including a set screw configured for securing the inner member in the hole and a means defining a threaded opening extending from the exterior of the outer member into the hole for adjustably receiving the set screw; in which the die holder inner member may be positioned relative to the outer member such that the exposed die surface is substantially parallel with the highest plane; in which the die holder inner member may be a cylinder, one surface being one end of the cylinder; in which the die holder outer member may also be a cylinder, the hole being substantially the same size as the diameter of the inner cylinder for receiving the inner cylinder for movement longitudinally and coaxially within the outer cylinder; and to accomplish the above objectives and purposes in an inexpensive, uncomplicated, durable, versatile, and reliable method and apparatus, inexpensive to manufacture, and readily suited to the widest possible utilization. 
    
    
     These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawings, and the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Certain embodiments of the present invention are described, reference being made to the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of an embodiment of the present invention. 
     FIG. 2 is a cross-sectional view of the FIG. 1 assembly taken on line  2 — 2  thereof. 
     FIG. 3 is a side view of the outer member shown in FIG. 1, showing a threaded hole. 
     FIG. 4 is a top view of the outer member shown in FIGS. 1 and 3. 
     FIG. 5 is a side view of the inner member shown in FIG. 1, showing a flattened side portion also shown in FIG.  6 . 
     FIG. 6 is a top view of the embodiment of the inner member shown in FIGS. 1 and 5. 
     FIG. 7 is a cross-sectional view taken on line  7 — 7  of FIG.  1 . 
     FIG. 8 is an exaggerated figurative drawing of the FIG. 1 assembly with portions of the invention omitted for clarity of illustration. 
     FIG. 9 is a figurative geometric illustration of the planes and angles defined by the several surfaces and elements of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawings, the new and improved method for holding and abrading a die, and the apparatus for carrying out the method according to the present invention, will now be described. 
     FIG. 1 shows a perspective view of the preferred embodiment of the present invention  100 , including an outer member  10 , an inner member  20 , a locking means  25 , a die  50 , an abrasive  200 , and a rotatable wheel  300 . 
     FIG. 3 shows a side view of the outer member  10  of the present invention. In the illustrated embodiment, the outer member  10  is a cylinder with four holes  11 ,  12 , and  13 . Hole  11  (FIG. 4) is a cylindrical receiving hole which passes longitudinally and coaxially through the outer member  10 . Hole  12  is a threaded set screw hole which passes through the side of the outer member  10  to the receiving hole  11 . Two threaded mount screw holes  13  are located on the bottom side of the outer member  10 . 
     FIG. 5 shows a side view of the preferred embodiment of the inner member  20  of the present invention. Member  20  is a cylinder with one threaded mount screw hole  21  located on its bottom side. 
     The outer member  10  (FIG. 7) entirely receives the inner member  20  into the receiving hole  11  such that the top surface  22  of the inner member  20  is parallel with the top surface  14  of the outer member  10 , and such that it is beneath the receiving hole  11  opening  15  of the outer member  10  surface  14  at a depth substantially as deep as the thickness of the die  50  which is to be delayered. A set screw  25  locks the inner member  20  at a predetermined position relative to the outer member  10 . The set screw extends through the set screw hole  12  from the exterior of the outer member  10  into the set screw hole  12  and abuts a flat portion  26  on the side of the inner member  20 . The inner member  20  is positioned such that when the die  50  is attached to its top surface  22 , the exposed to-be-abraded surface  51  of the die  50  is exposed at a predetermined exposure increment  32  (FIGS. 7 and 9) just outside the receiving hole  11  opening  15 . This predetermined increment  32  subtends an exposure angle  30  (FIG. 9) between the top of the highest point  52  of the exposed to-be-abraded die surface  51  and the highest plane  16  around the receiving hole  11  of the outer member  10 . The three highest points (not shown) on the outer member  10  define the highest plane  16 . The predetermined increment  32  is preferably adjusted to define an exposure angle  30  less than one degree. 
     In known fashion, an abrasive disk  200  (FIG. 8) then abrades the die  50  down to a desired die layer. A rotatable wheel  300  provides a firm and flat support for the abrasive disk  200  as the die  50  is applied by force against the abrasive disk  200 . The abrasive disk  200  (also referred to herein as the “abrasive”) is firm so that it does not bunch up and is pliable such that all portions of the die  50  contact the abrasive  200  and are delayered. If the rotatable wheel  300  is positioned such that it faces upward, with the die  50  facing and being forced downward, a liquid slurry with acid can be used in place of an abrasive disk  200 . In either case, because all portions of the die surface  51  are exposed to abrading action, the factor principally causing the variance in the abrading rate is the force that is applied to any given point on the die  50  by the abrasive  200 —the greater the force, the faster the delayering rate. 
     The wheel angle  31  (FIG. 9) is the angle between the highest plane  16  of the top surface  14  of the outer member  10  and the rotatable wheel  300 . Two features of the invention minimize the wheel angle  31 . These features are the outer member width  17  (FIG. 8) and the ability to control the exposure increment or degree of die exposure  32 . The outer member width  17  defines the distance  33  from the outer perimeter of the outer member  10  to the top of the die. The degree of die exposure  32  defines the distance  32  (FIG. 9) from the top of the die to the top surface  14  of the outer member  10 . The larger the ratio between the length of the outer member width  17  to the degree of die exposure  32  the smaller the wheel angle  31 . In this case, the outer member width  17  is sufficiently wide and the die exposure  32  is sufficiently small such that the wheel angle  31  is minimized such that the highest plane  16  of the top surface of the outer member  10  is essentially flat against the rotatable wheel to which it is applied. When the wheel angle  31  is minimized, any tilt due to the exposure angle  30  is also minimized. 
     Even with the wheel angle  31  essentially at zero degrees, with the outer member  10  flat against the rotatable wheel  300 , there will always be a small tilt angle component  40  (FIG. 8) added or subtracted to the die  50  relative to the rotatable wheel due to the inherent thickness and unevenness of the adhesive  45  which attaches the die  50  to the inner member  20  surface  22 . The adhesive  45  used is typically glue or wax. There is no current means of completely eliminating this tilt  40 . This tilt  40  thus ultimately defines the abrading angle  60  (FIG.  8 ). 
     The slight tilt  40  in the die  50  due to the adhesive  45  is actually desired. As mentioned above, when the die  50  is delayered using a rotatable wheel  300 , the conventional method, a rainbow appears on the die. The rainbow rings on the die  50  can adversely affect visual analysis of particular die circuits, specifically when the rings pass over and obscure transistors of interest. This is an undesired die surface characteristic. This rainbow effect is not a problem when there is sufficient distance between rainbow rings. While the required distance will vary with the size of the circuit to be analyzed, a distance of 10 microns will usually suffice; a distance of 1000 microns is ideal. To obtain the maximum distance, the abrading  60  angle must be minimized to, ideally, less than one degree. If the abrading angle  60  is at zero degrees, however, concentric circles appear, rather than the rainbow rings. These circles significantly interfere with-visual analysis. Also, the distances between such circles are too short and cannot be controlled. This undesired die characteristic is eliminated by the tilt  40 . The tilt  40  is substantially less than one degree and therefore enhances the quality of the entire analysis procedure. 
     The width  17  of the outer member  10  and the ability to recess the die into hole  11  essentially eliminates any wobbling that might occur during the delayering process. This allows the die to be abraded uniformly as it minimizes any pressure differential as the die is being delayered. As mentioned above, a die that is not delayered uniformly is problematic to the analysis because it causes multidirectional rainbow rings, too many of which impair visual analysis of the die. Uniform delayering provided by the current invention produces desired unidirectional rainbow rings. 
     The outer member  10  is small enough to allow the user to place the die sample  50  in other failure analysis tools without having to remove the die  50  from the die holder. As stated above, other tools include the optical microscope, scanning electron microscope, and plasma etcher. As stated above, undesired die surface characteristics result when the die is attached to the holder, delayered, detached from the holder, reattached, then delayered again. The present invention avoids these undesired die characteristics, because the die  50  can be intermittently used in different tools during the entire delayering process without ever changing its position relative to the die holder. For certain types of analysis equipment, such as the scanning electron microscope, the inner member  20  can be detached from the outer member  10  and mounted onto the scanning emission microscope via the mount screw hole  21  on the bottom of the inner member  20 . 
     As may be seen, therefore, the present invention provides numerous advantages. Principally, it eliminates problems and limitations resulting from instability, imprecision and lack of portability, problems that the prior art has failed to resolve satisfactorily. 
     Other variations on the present invention will occur after reading and understanding the present disclosure. One such change for example might be the use of a micrometer adjuster or loaded spring as the positioner. 
     Therefore, while the methods and forms of apparatus herein described constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to these precise methods and forms of apparatus, and that changes may be made therein without departing from the scope of the invention.

Technology Classification (CPC): 1