Patent Publication Number: US-11031358-B2

Title: Overhang model for reducing passivation stress and method for producing the same

Description:
TECHNICAL FIELD 
     The present disclosure relates to semiconductor fabrication. In particular, the present disclosure relates to reducing passivation stress within a fingerprint sensor. 
     BACKGROUND 
     Fingerprint sensing semiconductors are widely used in computing devices to support fingerprint scanning applications. The sensors are typically embedded under a display, behind a metal shell or within a frame of the computing device. Dual pad mask passivation is performed during the sensor fabrication process to protect it from moisture, corrosion and other external stimuli and increase their sensibility to touch. Per this approach, a thin passivation layer is deposited on top of a planarized passivation pattern formed over the semiconductor. Unfortunately, this passivation scheme often results in the formation of stress fractures in the most vulnerable areas of the thin upper passivation film, which in turn degrades the integrity of the passivation layer. 
     One approach for addressing this issue is to increase the upper passivation oxide thickness. However, the increased thickness reduces sensor performance and increases production cost. A need therefore exists for a device and method of forming a sensor with structural features to prevent passivation stress fractures with minimal cost. 
     SUMMARY 
     An aspect of the present disclosure is a method of forming a sensor with structural features to prevent passivation stress fractures with minimal cost. 
     Another aspect of the present disclosure is a sensor with increased overhang to prevent passivation stress fractures. 
     Additional aspects and other features of the present disclosure will be set forth in the description which follows and in part will be apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present disclosure. The advantages of the present disclosure may be realized and obtained as particularly pointed out in the appended claims. 
     According to the present disclosure, some technical effects may be achieved in part by a method including: forming a first passivation layer over a dielectric layer patterned over a first top metal layer of a logic region of a sensor and a second top metal layer of an array region of the sensor; planarizing the first passivation layer and the dielectric layer to form a level surface above the first top metal layer and the second top metal layer; etching the dielectric layer to form a pad opening in the array region of the sensor based on a predetermined overhang value, the pad opening exposing a portion of the surface of the second top metal layer; and forming a second passivation layer over the level surface and the pad opening in the array region. 
     Aspects of the present disclosure includes etching the second passivation layer and the dielectric layer under the second passivation layer to form a pad opening in the logic region of the sensor based on the predetermined overhang value, the pad opening exposing a portion of the surface of the first top metal layer. Further aspects include, the step of etching further including: depositing an etch mask over a portion of the second passivation layer, wherein the deposition is based on the dimensions of the pad opening in the logic region of the sensor. Another aspect includes the dimensions of the pad opening in the logic region are based on the predetermined overhang value, and wherein the predetermined overhang value is a measure of overlap between the first top metal layer and the second passivation layer from the pad opening. 
     Additional aspects include, the step of etching further including: depositing an etch mask over a portion of the dielectric layer and the first passivation layer, wherein the deposition is based on the dimensions of the pad opening in the array region of the sensor. Another aspect includes the dimensions of the pad opening in the array region are based on the predetermined overhang value, and wherein the predetermined overhang value is a measure of overlap between the second top metal layer and the second passivation layer from the pad opening. 
     Additional aspects include the predetermined overhang value is based on a correlation between a deformation shape of the first top metal layer or the second top metal layer and a deformation behavior of the second passivation layer leading to stress optimization in the second passivation layer. Another aspect includes the correlation is based on finite element analysis (FEA). Another aspect includes the dielectric layer which includes a high-density plasma (HDP) oxide, a silicon or doped silicon-based low-K dielectric. Further aspect includes the first passivation layer which includes tetraethoxysilane (TEOS) and the second passivation layer which includes a HDP oxide and nitride. 
     Another aspect of the present disclosure is a device including: a first top metal layer within a logic region of a sensor; a second top metal layer within an array region of the sensor; a dielectric layer over the logic region and the array region, height of the dielectric layer over the first and second top metal layers greater than the height of the dielectric layer in-between and around the first and second top metal layers; a first passivation layer over the dielectric layer in-between and around the first and second top metal layers, height of the first passivation layer level with the height of the dielectric layer over the first and second top metal layers; a pad opening in the logic region of the sensor with dimensions based on a predetermined overhang value, the pad opening exposing the first top metal layer; a pad opening in the array region of the sensor with dimensions based on the predetermined overhang value; and a second passivation layer over the dielectric layer, the first passivation layer and the pad opening in the array region of the sensor. 
     Aspects of the device include a via connected to a bottom surface of the first top metal layer, the via interconnecting the sensor to a logic device; and a via connected to a bottom surface of the second top metal layer, the via interconnecting the sensor to an array, wherein the dielectric layer is over the via, and wherein the dielectric layer includes HDP oxide, silicon or doped silicon-based low-K dielectric. Another aspect includes an etch mask over a portion of the dielectric layer and the first passivation layer, wherein the portion is based on the dimensions of the pad opening in the array region of the sensor. A further aspect includes an etch mask over a portion of the second passivation layer, wherein the portion is based on the dimensions of the pad opening in the logic region of the sensor. Other aspects include the predetermined overhang value is a measure of overlap between the second top metal layer and the second passivation layer from the pad opening in the array region. A further aspect includes the predetermined overhang value is a measure of overlap between the first top metal layer and the second passivation layer from the pad opening in the logic region. Additional aspects include, the first passivation layer which includes TEOS and the second passivation layer which includes HDP oxide and nitride. 
     A further aspect of the present disclosure is a method including: determining a correlation between a deformation shape of a top metal layer and a stress factor of an upper passivation layer of a sensor; and determining an overhang value based on the correlation, wherein the dimensions of a pad opening to be etched over a top metal layer in a region of a sensor is based on the overhang value. 
     Aspects of the present disclosure include the correlation based on FEA. Another aspect includes the predetermined overhang value is a measure of overlap between the top metal layer and a second passivation layer from the pad opening. 
     Additional aspects and technical effects of the present disclosure will become readily apparent to those skilled in the art from the following detailed description wherein embodiments of the present disclosure are described simply by way of illustration of the best mode contemplated to carry out the present disclosure. As will be realized, the present disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  (Background Art) schematically illustrates a cross-sectional view of a passivation scheme for a sensor device, in accordance with an exemplary embodiment; 
         FIGS. 2A through 2E  schematically illustrate a process flow for forming a sensor with increased overhang to prevent passivation stress fractures, in accordance with exemplary embodiments; 
         FIGS. 3A and 3B  schematically illustrate a cross-sectional view of a passivation scheme for a sensor having increased overhang, in accordance with exemplary embodiments; and 
         FIGS. 3C and 3D  schematically illustrate a top view of a passivation scheme for a sensor having increased overhang, in accordance with an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of exemplary embodiments. It should be apparent, however, that exemplary embodiments may be practiced without these specific details or with an equivalent arrangement. In other instances, well-known structures and devices are shown in block diagram form to avoid unnecessarily obscuring exemplary embodiments. In addition, unless otherwise indicated, all numbers expressing quantities, ratios, and numerical properties of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” 
     The present disclosure addresses and solves the problem of preventing stress fractures from occurring in a passivation layer applied to a sensor device during device fabrication. Current sensors, such as those for supporting fingerprint scanning and other applications, require dual pad mask passivation. As depicted by way of example in the cross-sectional sensor view of  FIG. 1  (Background Art), dual pad mask passivation typically requires a passivation layer  101  (e.g., silicon nitride (SiN)) to be deposited on top of a planarized passivation pattern (e.g., a HDP oxide such as silicon oxide (SiO or SiO 2 )) formed over the various elements of sensor. The passivation layer  101  thus serves as an insulating, protective layer to prevent mechanical and chemical damage during assembly and packaging of the sensor. 
     Unfortunately, the thin (upper) passivation layer  101  deposited on the silicon oxide  103  is subject to much stress, which particularly occurs in a vulnerable area  105  of the passivation layer(s). This further leads to the formation of cracks/fractures  107  that degrade the integrity of the passivation layer and limit the performance of the sensor (e.g., receptivity to touch, sensibility). 
     This problem is solved, inter alia, by forming a sensor with increased overhang to prevent passivation stress fractures with minimal cost/manufacturing variation. As will be discussed later herein, increased overhang reduces the amount of stress that occurs in the most vulnerable sections, i.e., nitride bend/corner area  105  of the passivation layer. By way of example, overhang is an amount of overlapping area between a top metal layer  111  and a passivation layer. Thus, an overhang value  109  may be determined and/or expressed as a distance from the edge of the top/second applied passivation layer  101  from a pad opening  113  to the edge of a top metal layer  111  of the sensor. 
     Methodology in accordance with embodiments of the present disclosure includes forming a first passivation layer over a dielectric layer patterned over a first top metal layer of a logic region of a sensor and a second top metal layer of an array region of the sensor. The first passivation layer and the dielectric layer are planarized to form a level surface above the first top metal layer and the second top metal layer. The dielectric layer is etched to form a pad opening in the array region of the sensor based on a predetermined overhang value, the pad opening exposing a portion of the surface of the second top metal layer; and a second passivation layer is formed over the level surface and the pad opening in the array region. 
     Still other aspects, features, and technical effects will be readily apparent to those skilled in this art from the following detailed description, wherein preferred embodiments are shown and described, simply by way of illustration of the best mode contemplated. The disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
       FIGS. 2A through 2E  schematically illustrate a process flow for forming a sensor with increased overhang to prevent passivation stress fractures, in accordance with exemplary embodiments. The process flow for the passivation scheme is applied to a sensor device  200  as depicted in  FIG. 2A . 
     In  FIG. 2A , the sensor  200  is a semiconductor structure that includes a logic region  201  and array region  203 . The logic region  201  corresponds to a function of the sensor  200  for interacting with logic elements (not shown for illustrative convenience) of an underlying semiconductor. An interconnection between the sensor  200  and an underlying logic element is formed by way of a via  207  that connects to a first top metal layer  209 . The first top metal layer  209  serves as a sensing element or electrode, such as in the case of capacitive touch detection. 
     The array region  203  corresponds to a function of the sensor  200  for interacting with an array of other sensors (not shown for illustrative convenience). An interconnection between the sensor  200  and the underlying array is formed by way of via  211  and second top metal layer  213 . The second top metal layer  213  serves as a sensing element or electrode, such as in the case of capacitive touch detection. It is contemplated, in certain embodiments, that the first and second top metal layers  209  and  213  respectively are formed as metal-insulator-metal structures includes various types of metal (e.g., aluminum) and/or capacitive elements. Still further, it is contemplated via  207  and  211  and respective top metal layers  209  and  213  may be formed according to known Damascene processing or back end of line (BEOL) processing techniques. 
     The first top metal layer  209  and second top metal layer  213  and their respective vias  207  and  211  are surrounded by a HDP oxide layer  215 . In certain embodiments, the HDP oxide layer  215  is a dielectric layer and is formed as a silicon or doped silicon-based low-K dielectric, e.g., SiO or SiO 2 , for separating the respective functions of the sensor  200 . Hence, the logic region  201  may support interfacing of the sensor with corresponding sense perception (per array region  203 ) and/or determination logic (per logic region  201 ). Of note, the HDP oxide layer  215  may be patterned accordingly, per known etching and masking techniques, such that the height of the HDP oxide layer is greater over the first and second top metal layers  209  and  213  respectively. 
     Per the exemplary passivation scheme, a first passivation layer  217  is formed over the HDP oxide layer  215  as patterned over the first top metal layer  209  of the logic region  201  and the second top metal layer  213  of the array region  203 . By way of example, the first passivation layer  217  includes TEOS, TEOS ozone oxide or any other known material for supporting chemical-mechanical planarization. In another process step, the first passivation layer  217  and the HDP oxide layer  215  are planarized to form a level surface of the sensor  200 , as depicted in  FIG. 2B . Per this step, the height of the first passivation layer  217  and HDP oxide layer  215  above the surface of the first top metal layer  209  and second top metal layer  213  are identical. 
     In  FIG. 2C , the HDP oxide layer  215  is etched to form a pad opening  219  in the array region  203  of the sensor  200  based on a predetermined overhang value. The pad opening  219  therefore exposes a portion of the surface of the second top metal layer  213  for enabling the sensing element or electrode to actively detect external stimuli (i.e., touch). In the exemplary embodiment, the dimensions of the pad opening  219  are defined based on the predetermined overhang value. As such, the surface area exposed per the pad opening  219  is variable relative to the fixed dimensions of the second top metal layer  213  of the array region  203 . 
     In certain embodiments, the overhang value is a measure of overlap between the second top metal layer  213  and a second passivation layer (e.g., layer  225  as shown in  FIG. 2D ) from the pad opening. The overhang value may be expressed as a distance (in micrometers) from the pad opening  219  to the edge of the second top metal layer  213 , as an area value or area differential (e.g., area of the second top metal layer  213  minus the area of the pad opening  219 ), or the like. Moreover, the overhang value may be predetermined as an optimal value ahead of the sensor fabrication process based on FEA (e.g., per finite element methodology (FEM) or modeling) of known characteristic and/or response set data related to the passivation layer(s), the top metal layer(s) and other elements of the device  200 . Per FEA or FEM, material characteristics, design requirements, and other elements of the sensor may be observed and correlated. 
     For example, the predetermined overhang value may be based on a correlation between a deformation shape of a given top metal layer and the second passivation layer resulting in an optimized stress factor (as observed in a nitride bend/corner area  105  of a second passivation layer). Based on this, an optimal value or range of values (e.g., 3-14 μm) for the overhang distance may be determined and adapted for different passivation schemes and sensor  200  types. It is noted, therefore, that the overhang value informs the etching process to ensure exact patterning of the HDP oxide layer  215  for formation of a pad opening  219  per any known etching means. 
     The etch process step depicted in  FIG. 2C  may further include performing a masking process. In this process, an etch mask is deposited over a portion of the HDP oxide layer  215  and the first passivation layer  217  prior to etching. Selective deposition of the etch mask ensures that only the portions of the sensor  200  that are not to be etched get masked, which is based on the dimensions of the pad opening  219  in the array region  203 . Hence, the predetermined overhang value informs the masking process during device fabrication for specifying where the mask is, and is not, to be applied. 
     It is noted that the shape of the pad opening  219  as shown is for illustrative purposes only. The shape, size and general dimensions of the pad region  219  may vary as defined based on the predetermined overhang value. Per the process flow described in  FIG. 2C , the resulting sensor  200  includes a level surfacing of the first passivation layer  217  and HDP oxide layer  215  with a single pad opening  219  in the array region  203 . 
     In  FIG. 2D , a second passivation layer  225  is formed over the level surface of both regions  201  and  203  and the pad opening  219  in the array region  203 . The second passivation layer  225  is formed (deposited) as a HDP oxide layer  221  with an additional nitride layer  223  formed atop the HDP oxide layer  221 . As the uppermost passivation layer, the second passivation layer  225  is of a thickness and material composition suitable for protecting the sensor  200 , including the surface of the sense element/electrode (second top metal layer  213 ), from degradation due to external stimuli, i.e., contamination, moisture, etc. 
     In  FIG. 2E , the second passivation layer  225  and HDP oxide layer  215  under the second passivation layer  225  are etched to form a pad opening  227  in the logic region  201  of the sensor  200  based on the predetermined overhang value. The pad opening  227  therefore exposes a portion of the surface of the first top metal layer  209  for enabling the sensing element or electrode to actively detect external stimuli (i.e., touch). As described previously with respect to pad opening  219 , the dimensions of the pad opening  227  are defined based on the predetermined overhang value. As such, the surface area exposed per the pad opening  227  is variable relative to the fixed dimensions of the first top metal layer  209  of the logic region  201 . It is contemplated, in certain embodiments, that the predetermined overhang value for the array region  203  and logic region  201  may be identical or different depending on design and manufacturing requirements. 
     The etch process step depicted in  FIG. 2E  may further include performing a masking process. In this process, an etch mask is deposited over only the portion of the second passivation layer  225  not to be etched, i.e., no mask applied to the area of the second passivation layer  225  for patterning the pad opening  227 . As described previously with respect to pad opening  219 , selective masking is performed based on the dimensions of the pad opening  227 . 
     Per the process flow described in  FIG. 2E , the resulting sensor  200  includes dual pads, as defined by per pad openings  219  and  227 . Furthermore, the pad opening  219  in the array region  203  has formed thereon the second passivation layer  225  while the pad opening  227  in the logic region  201  is fully exposed. In both regions  201  and  203 , the respective pad openings  227  and  219  are formed based on the predetermined overhang value, thus preventing the occurrence of stress fractures within the second passivation layer  225 . Of note, the process flow described herein enables the passivation stress response of the sensor  200  to be predetermined, planned and adapted for via overhang design and without any change in the manufacturing process. 
       FIGS. 3A and 3B  schematically illustrate a cross-sectional view of a passivation scheme for a sensor having increased overhang, in accordance with exemplary embodiments. In  FIG. 3A , the second passivation layer  305  includes a HDP oxide layer  303  and nitride layer  301  formed over a top metal layer  307  of a sensor and a planarized, first passivation layer  309  formed of TEOS. The second passivation layer  305  is also formed over a pad opening  311   a . The distance  313   a  from the pad opening  311   a  to the edge of the top metal layer  307 , referred to herein as overhang, is lengthened. 
     A top view of the passivation scheme depicted above, with lengthened overhang  313   a , is shown in  FIG. 3C . In  FIG. 3C , the second passivation layer  305  (formed over the pad opening  311   a ) overlaps the top metal layer (M5 layer)  307 . The dimensions of the pad opening  311   a  are defined by the overhang value  313   a  as measured in micrometers (μ). In this example, the overhang value is equal on each overlapping side. Alternatively, the overhang may be measured in square units (e.g., μ 2 ) for representing an area differential between the top metal layer  307  and the area of pad opening  311   a . Still further, in an alternate embodiment, the overhang value may be different on different overlapping sides. In the latter scenario, multiple predetermined overhang values may be selected in advance of the sensor fabrication process per FEA. 
     In  FIG. 3B , the distance  313   b  from the pad opening  311   a  to the edge of the top metal layer  307  is lengthened. This is depicted in the corresponding top view of  FIG. 3D , which shows increased dimensions of the pad opening  311   b  as defined by longer overhang value  313   b . The increased overhang minimizes the stress response at the bend/corner area  315  of the second passivation layer  305 . This contrasts with the overhang value  109  of  FIG. 1  (Background Art), wherein the shorter distance increases the amount of stress that occurs at a bend/corner area  105 . It is noted, in this example, that the increased overhang  313   b  corresponds to decreased dimensions and/or area of the pad opening  311   b . As such, the reduced size of the opening may be accounted for in advance, i.e., based on the predetermined overhang value, to support sensor design and planning. 
     The embodiments of the present disclosure can achieve several technical effects including preventing stress fractures from occurring in a passivation layer formed over a sensor. Furthermore, the present disclosure enables a predetermined (optimal) overhang value or range to be determined for planning, designing and manufacturing a sensor. Still further, the present disclosure supports the development of sensors with increased overhang with no adverse impact to cost or sensor performance. Devices formed in accordance with embodiments of the present disclosure enjoy utility in various industrial applications, e.g., microprocessors, smart phones, mobile phones, cellular handsets, set-top boxes, DVD recorders and players, automotive navigation, printers and peripherals, networking and telecom equipment, gaming systems, and digital cameras. The present disclosure enjoys industrial applicability in any of various types of semiconductor devices including MRAMs, ReRAMs and FeRAMs. 
     In the preceding description, the present disclosure is described with reference to specifically exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present disclosure, as set forth in the claims. The specification and drawings are, accordingly, to be regarded as illustrative and not as restrictive. It is understood that the present disclosure is capable of using various other combinations and embodiments and is capable of any changes or modifications within the scope of the inventive concept as expressed herein.