Abstract:
A method for wafer alignment includes forming a first alignment circuit within a first semiconductor wafer; the first alignment circuit is configured to emit an optical signal. Next, the first alignment circuit is activated upon receiving a first activation signal from a wafer bonding tool then the optical signal is sent to a second alignment circuit in a second semiconductor wafer in overlapping relation to the first semiconductor wafer. The second alignment circuit transmits a second activation signal to the wafer bonding tool and consequently the wafer bonding tool initiates an alignment technique between the first and second semiconductor wafers. The alignment technique uses the first and second alignment circuits for optical alignment.

Description:
BACKGROUND 
     The present invention generally relates to wafer alignment for wafer bonding in 3-dimensional (3D) integration processes, and more particularly, to wafer-to-wafer alignment using light emitting diode (LED) and light sensing diode (LSD) devices. 
     Wafer bonding is a technology used in microelectronics fabrication, in which a first semiconductor wafer having first chip structures may be aligned with second chip structures on a surface of a second semiconductor wafer for fabricating an electronic circuit. The contact may be arranged in such a way that signals may be transported from at least one first chip structure on the first semiconductor wafer to at least one second chip structure on the second semiconductor wafer and vice versa. This arrangement is often referred to as a 3D wafer alignment. 
     Typically, wafer-to-wafer alignment for wafer bonding may be accomplished through complicated alignment techniques that rely on geometric transpositions of passive structures that represent geometric coordinates on one wafer such that a minimization of an alignment error may be accommodated through external measurement analysis and feedback instrumentation. Such instrumentation relies on optical measurement and detection sensors that predominantly operate in the infrared (IR) or near IR range of the electromagnetic spectrum. 
     Multiple sources of error may be inherent in this range of the electromagnetic spectrum, such as: refraction of image signal due to possible non-optical linearity of the substrate, opacity of substrate due to metal masking layers, intrinsic error in accuracy due to IR wavelength, etc. As semiconductor technology continues the trend of miniaturization, high-precision wafer-to-wafer alignment becomes increasingly important for 3D integration processes. 
     SUMMARY 
     According to one embodiment of the present disclosure, a method for wafer alignment, includes: forming a first alignment circuit within a first semiconductor wafer, the first alignment circuit configured to emit an optical signal, activating the first alignment circuit upon receiving a first activation signal from a wafer bonding tool, sending the optical signal to a second alignment circuit in a second semiconductor wafer in overlapping relation to the first semiconductor wafer, the second alignment circuit transmitting a second activation signal to the wafer bonding tool and the wafer bonding tool initiating an alignment technique between the first and second semiconductor wafers, the alignment technique using the first and second alignment circuit for optical alignment. 
     According to another embodiment of the present disclosure, a method for wafer alignment, includes: forming a first optical device within a first semiconductor wafer, the first optical device configured to emit an optical signal, forming a first antenna within the first semiconductor wafer, the first antenna configured to receive a first activation signal, forming a first discriminator circuit within the first semiconductor wafer, the first discriminator circuit communicating with the first antenna for detecting the first activation signal and activating the first optical device upon receiving the first activation signal, forming a second optical device within a second semiconductor wafer, the second optical device configured to receive an optical signal from the first semiconductor wafer and transmit a second activation signal upon receiving the optical signal, forming a second antenna within the second semiconductor wafer, the second antenna configured to transmit the second activation signal, forming a second discriminator circuit within the second semiconductor wafer, the second discriminator circuit communicating with the second antenna for emitting a second activation signal and initiating an alignment technique between the first and second semiconductor wafers using the first and second optical devices. 
     According to another embodiment of the present disclosure, a wafer alignment structure includes: a first alignment circuit located within a first semiconductor wafer including a first optical device connected to a first antenna positioned within a back-end-of-the-line region of the first semiconductor wafer and to a first discriminator circuit positioned within a front-end-of-the-line region of the first semiconductor wafer, the first semiconductor wafer being located within a first bonding chuck of a wafer bonding tool and a second alignment circuit located within a second semiconductor wafer including a second optical device connected to a second antenna positioned within a back-end-of-the-line region of the second semiconductor wafer and to a second discriminator circuit positioned within a front-end-of-the-line region of the second semiconductor wafer, the second semiconductor wafer being positioned within a second bonding chuck of a wafer bonding tool. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The following detailed description, given by way of example and not intended to limit the invention solely thereto, will best be appreciated in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view of two semiconductor wafers including alignment circuits depicting an alignment state between the two semiconductor wafers, according to an embodiment of the present disclosure; 
         FIG. 2  is a top view of two semiconductor wafers depicting a location for the alignment circuits within a surface of the semiconductor wafers, according to an embodiment of the present disclosure; 
         FIG. 3  is a top view of two semiconductor wafers depicting an alternate location for the alignment circuits within the surface of the semiconductor wafers, according to an embodiment of the present disclosure; 
         FIG. 4  is a cross-sectional view of two semiconductor wafers including alignment circuits depicting a misalignment state between the two semiconductor wafers, according to an embodiment of the present disclosure; and 
         FIG. 5  is a flow chart depicting a method for the fabrication of alignment circuits within a semiconductor wafer, according to an embodiment of the present disclosure. 
     
    
    
     The drawings are not necessarily to scale. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention. In the drawings, like numbering represents like elements. 
     DETAILED DESCRIPTION 
     Exemplary embodiments now will be described more fully herein with reference to the accompanying drawings, in which exemplary embodiments are shown. This invention may, however, be modified in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of this invention to those skilled in the art. In the description, details of well-known features and techniques may be omitted to avoid unnecessary obscuring the presented embodiments. 
     One method of wafer-to-wafer alignment is described in detail below by referring to the accompanying drawings in  FIGS. 1-4 , in accordance with an illustrative embodiment of the present disclosure. 
       FIG. 1  is a cross-sectional view of two semiconductor wafers  100  and  200  positioned within a wafer bonding tool  300 , each wafer having an alignment circuit, according to an embodiment of the present disclosure. More specifically,  FIG. 1  depicts a cross-sectional view of a first semiconductor wafer  100  located within a first bonding chuck  130  of the wafer bonding tool  300  and a second semiconductor wafer  200  positioned within a second bonding chuck  230  of the same wafer bonding tool  300 . The second bonding chuck  230  containing the second semiconductor wafer  200  may be located on top of the first bonding chuck  130  containing the first semiconductor wafer  100 . 
     It should be noted that a top surface of the semiconductor wafer  200  is positioned such that it is directly opposite from a top surface of the semiconductor wafer  100 . 
     With continued reference to  FIG. 1 , the first semiconductor wafer  100  may include a first alignment circuit  302 . The first alignment circuit  302  may include a first antenna  110  formed within a back-end-of-the-line (BEOL) region of the first semiconductor wafer  100  for receiving a first activation signal. The first antenna  110  may be electrically connected to a first discriminator circuit  106 . The first discriminator circuit  106  may be formed within a front-end-of-the-line (FEOL) region of the first semiconductor wafer  100 . The first antenna  110  may communicate the first activation signal to the first discriminator circuit  106  to activate a first optical device  102  located within the first alignment circuit  302 . The first optical device  102  may be electrically connected to the first discriminator circuit  106  and the first antenna  110 . In an embodiment of the present disclosure, the first optical device  102  may include a light emitting diode (LED) device. 
     According to the configuration of the first alignment circuit  302  described above, the first optical device  102  (hereafter referred to as “LED device”) may be remotely activated to begin emitting the optical signal from the first semiconductor wafer  100 . The activation process including: the first antenna  110  detecting the first activation signal originated in the wafer bonding tool  300 , and then communicating the first activation signal to the first discriminator circuit  106  which in turn may activate the LED device  102 . In another embodiment of the present disclosure, the LED device  102  may be activated by connecting the alignment circuit  302  to a wired circuit formed within the semiconductor wafer  100 . 
     The LED device  102  may be positioned within the same area as the first antenna  110  to preserve area within the first semiconductor wafer  100 . 
     With continued reference to  FIG. 1 , the second semiconductor wafer  200  may include a second alignment circuit  304 . The second alignment circuit  304  may include a second antenna  210  located within a back-end-of-the-line (BEOL) region of the second semiconductor wafer  200  for transmitting a second activation signal. The second antenna  210  may be electrically connected to a second discriminator circuit  206 . The second discriminator circuit  206  may be formed within a front-end-of-the-line (FEOL) region of the second semiconductor wafer  200 . A second optical device  202  may be formed within the second alignment circuit  304 . The second optical device  202  may be electrically connected to the second discriminator circuit  206  and the second antenna  210 . In an embodiment of the present disclosure, the second optical device  202  may include a light sensing diode (LSD) device. The second discriminator circuit  206  may detect a second activation signal, also referred to as a current signal, sent from the second optical device  202  (hereafter referred to as “LSD device”) and transmit the second activation signal through the second antenna  210  to an external structure (not shown) located in the wafer bonding tool  300 . 
     The LSD device  202  may also be positioned within the same area as the second antenna  210  to preserve area within the second semiconductor wafer  200 . 
     It should be noted that the semiconductor wafers  100  and  200  may have multiple alignment circuits  302  and  304  used to achieve varying degrees of accuracy as will be described in detail below. 
     In an embodiment of the present disclosure, the configuration of the first alignment circuit  302  and the second alignment circuit  304 , may allow for the functioning of either optical device as an LED device or an LSD device by assigning the desired function through the discriminator circuits  106  and  206 . This feature may allow redundancy within the alignment circuits which may be functional in the case of having defective optical devices. In this embodiment, the alignment circuits  302  and  304  may include substantially similar alignment circuits including optical devices configured as LED devices  102  or LSD devices  202  within the first semiconductor wafer  100  and the second semiconductor wafer  200 . 
     The first alignment circuit  302  and the second alignment circuit  304  may be positioned in a kerf area of the first semiconductor wafer  100  and the second semiconductor wafer  200 , respectively. The kerf area, also referred to as a dicing channel, is an area between chip structures located on a semiconductor wafer as described below in  FIG. 2 . 
     With continued reference to  FIG. 1 , according to an embodiment of the present disclosure, the semiconductor wafers  100  and  200  may include first pinhole  112  and second pinhole  212 , respectively. The first pinhole  112  may be formed above the first alignment circuit  302  while the second pinhole  212  may be formed above the second alignment circuit  304 . In an embodiment of the present disclosure, the first and second pinholes  112 ,  212  may be formed by selective patterning and metallization. The patterning and metallization of pinholes  112 ,  212  may be conducted simultaneously with the formation of ordinary metal levels for an integrated circuit structure. 
     It should be noted that pinholes of different diameter may be positioned above a single alignment circuit. Depending on the diameter of the pinholes  112 ,  212 , a course or a fine alignment may be achieved. Multiple alignment circuits per wafer may have different pinhole diameter to achieve different levels of accuracy. Further, alignment circuits with different pinhole diameter may be used during an alignment procedure to identify misalignment direction and applying the corresponding corrections. 
     The diameter of the pinholes  112 ,  212  may be calibrated to form pinholes with gradated diameter (hereafter referred to as “gradated pinholes”). The optical signal may be transmitted from the first alignment circuit  302  in the first semiconductor wafer  100  to the second alignment circuit  304  to the second semiconductor wafer  200  through the first and second gradated pinholes  112  and  212 . The diameter of the gradated pinholes  112 ,  212  may correspond to the accuracy or tolerance of the desired alignment between wafers. 
     With continued reference to  FIG. 1 , the second semiconductor wafer  200  is shown in a state of complete alignment with the first semiconductor wafer  100 . In 3D integration processes, if two wafers are substantially aligned then they may be bonded together. In one embodiment, when the two semiconductor wafers  100  and  200  may be substantially aligned, the optical signal received by the LSD device  202  may be maximum. Next a second activation signal containing an alignment information may be transmitted from the LSD device  202  through the second discriminator circuit  206  to the second antenna  210 . The second antenna  210  may communicate the alignment information to the wafer bonding tool  300  to start a wafer bonding process. 
     It should be noted that although only one alignment circuit per wafer is depicted in  FIG. 1 , several alignment circuits may be positioned within the first and second semiconductor wafers  100 ,  200  as shown in  FIGS. 2-3  below. 
       FIG. 2  depicts top views of the first semiconductor wafer  100  and the second semiconductor wafer  200 . In this embodiment, the first semiconductor wafer  100  may include a plurality of chip structures  120  distributed within a surface of the first semiconductor wafer  100  according to a specific design. A section view of an area  122  of the first semiconductor wafer  100  depicts a possible location of the first alignment circuit  302 . The first alignment circuit  302  may be located in the kerf area  124  of the semiconductor wafer  100  around each of the chip structures  120 . However, the alignment circuit  302  may be located in the kerf area between any of the chip structures  120 . 
     Similarly, the second semiconductor wafer  200  may include a plurality of chip structures  220  distributed within the surface of the second semiconductor wafer  200 . A section view of an area  222  of the second semiconductor wafer  200  depicts a possible location of the second alignment circuit  304 . The second alignment circuit  304  may be located in the kerf area  224  of the second semiconductor wafer  200  around each of the chip structures  220 . As shown in  FIG. 2 , the distribution of the second alignment circuit  304  within the second semiconductor wafer  200  may be performed in a way such that for every first alignment circuit  302  within the first semiconductor wafer  100  there is a corresponding second alignment circuit  304  within the second semiconductor wafer  200 . 
     In one embodiment of the present disclosure, the alignment circuit distribution shown in  FIG. 2  includes positioning a first alignment circuit  302  in the kerf area between the chip structures  120  within the first semiconductor wafer  100  and positioning a second alignment circuit  304  in the kerf area between the chip structures  220  within the second semiconductor wafer  200 . This distribution may minimize global wafer-to-wafer misalignment on a chip-to-chip alignment level. Alternatively, in another embodiment of the present disclosure, the first and second alignment circuits  302  and  304  may be formed in the kerf area of a predetermined region of the first and second semiconductor wafers  100  and  200 . 
     In another embodiment, the alignment circuits  302 ,  304  may be located within a chip structure or in an edge area of each semiconductor wafer. 
       FIG. 3  shows an alternate configuration for positioning the alignment circuits  302  and  304  within the first and second semiconductor wafers  100  and  200 . In an embodiment of the present disclosure, a hybrid distribution may be considered. The hybrid distribution may include uniformly distributing both types of alignment circuits containing LED emitters and LSD receivers within the first semiconductor wafer  100  and the second semiconductor wafer  200 . 
     A detailed view of area  122  illustrates a possible hybrid distribution within the first semiconductor wafer  100 . Similarly, a detailed view of area  222  illustrates a possible hybrid distribution within the second semiconductor wafer  200 . In an embodiment of the present disclosure, for every first alignment circuit  302  there is a corresponding second alignment circuit  304  within the opposite semiconductor wafer. Each semiconductor wafer may have a combination of LED and LSD devices assigned to each chip structure uniquely. 
     In one embodiment, not all the kerf areas  124 ,  224  around the chip structures  122 ,  222  may include an alignment circuit as shown in  FIG. 3 . An example of hybrid configurations where only a portion of the chip structures may have alignment circuits include: a cross, a spiral and other geometric shapes (not shown). In another embodiment, the alignment circuits  302  and  304  may be positioned within the chip structures or in an edge area of each semiconductor wafer. 
     In some embodiments, conventional alignment marks (not shown) may exist in the first semiconductor wafer  100  and the second semiconductor wafer  200 . The presence of the alignment marks typically used in wafer alignment may facilitate an initial coarse alignment of the first and second semiconductor wafers  100 ,  200 . 
       FIG. 4  depicts a misalignment state between the first semiconductor wafer  100  and the second semiconductor wafer  200 . More particularly,  FIG. 4  shows the first and the second semiconductor wafers  100  and  200  after the second semiconductor wafer  200  moves with respect to the first semiconductor wafer  100  in the direction of arrow  400 . Alternatively, the second semiconductor wafer  200  may also be moved in the opposite direction of arrow  400 . The direction of arrow  400  may be essentially parallel to the first and second semiconductor wafers  100  and  200 . 
     In the misalignment state, the optical signal, represented by arrows in  FIG. 4 , may be deviated from a pathway provided by the second gradated pinhole  212 . In this case, a weakened optical signal may be detected by the LSD device  202  indicating that further adjustments may be needed to improve alignment between the first semiconductor wafer  100  and the second semiconductor wafer  200 . In consequence, the LSD device  202  may transmit the second activation signal through the second antenna  210  to an external structure (not shown) positioned in the wafer bonding tool  300  to initiate an alignment technique between the first and second semiconductor wafers. The second activation signal may contain information regarding a current alignment state between the first semiconductor wafer  100  and the second semiconductor wafer  200 . Once the second activation signal is transmitted to the alignment tool, the alignment information is analyzed and the position of the first and the second bonding chucks containing the first and second semiconductor wafers  100  and  200  is adjusted until the optical signal detected by the LSD device  202  is maximized. 
     According to the diameter of the second gradated pinhole  212 , a fine alignment or a coarse alignment may be performed. The course alignment may include gradated pinholes with a bigger diameter while the fine alignment may include gradated pinholes with a smaller diameter. In the coarse alignment case, second gradated pinholes  212  having a bigger diameter may provide a wider pathway for the optical signal to pass and be detected by the LSD device  202 . In this embodiment, the LSD device  202  may detect a stronger optical signal with less position adjustments between the first and second semiconductor wafers  100  and  200 . However, the first and second semiconductor wafers  100  and  200  may not be completely aligned. 
     In the fine alignment case, second gradated pinhole  212  having a smaller diameter may provide a narrower pathway for the optical signal to pass and be detected by the LSD device  202 , hence further position adjustments between the first and second semiconductors wafers  100  and  200  may be needed to obtain a stronger optical signal, this in turn may present better accuracy for aligning the first and second semiconductor wafers  100  and  200 . 
     It should be noted that although only one alignment circuit per semiconductor wafer is depicted in  FIG. 4 , several alignment circuits may be positioned within the first and second semiconductor wafers  100 ,  200  as shown in  FIGS. 2-3  above. 
       FIG. 5  is a flowchart indicating process steps for an embodiment of the present disclosure. In process step  502 , the first and second semiconductor wafers  100 ,  200  may be inserted within the corresponding bonding chuck of the wafer bonding tool  300  (as shown in  FIG. 1 ). In process step  504  the bonding tool  300  may send an activation signal detected by the first antenna  110  to activate the LED device  102  and begin emitting an optical signal. In process step  506  the LSD device may receive the optical signal and transmit an electrical signal with the alignment information through the second antenna  210  to an external structure located in the wafer bonding tool  300 . In processing step  508 , the wafer bonding tool  300  may analyze the received alignment information and then adjust the position of the bonding chucks containing the semiconductor wafers accordingly. In process step  510  a check may be performed to determine if the alignment specifications are met. If the alignment may be considered correct, then the process may end and the wafers may be bonded. If the alignment may not be considered correct, then another signal containing new alignment correction data may be transmitted to the first antenna  110  to repeat the alignment process from process step  504 . Depending on the embodiment, the first semiconductor wafer  100 , the second semiconductor wafer  200  or both may be adjusted in response to the alignment correction data determined in process step  508 . Process steps  504 - 510  may be repeated numerous times in an iterative manner until the semiconductor wafers are determined to be aligned. 
     The method described above may provide an alternate approach to traditional wafer alignment techniques. According to embodiments of the present disclosure, the need for external infrared alignment and alignment markers may be eliminated by using alignment circuits containing LED and LSD devices. The use of active and passive devices such as LED devices and LSD devices may enable self-alignment of the semiconductor wafers with respect to each other. This in turn may eliminate the need for bonding chuck calibrations. Additionally, the unique use of LED and LSD devices on a chip-level may minimize measurement error and may enable an advanced and more accurate alignment during wafer-to-wafer-bonding. 
     It may be noted that not all advantages of the present invention are include above. The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiment, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.