Patent Publication Number: US-2022223622-A1

Title: Memory structure and method of forming the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of U.S. provisional applications Ser. No. 63/136,646, filed on Jan. 13, 2021. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND 
     Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductor layers over a semiconductor substrate, and patterning the various material layers using lithography and etching techniques to form circuit components and elements thereon. 
     The semiconductor industry continues to improve the integration density of various electronic components (e.g., transistors, diodes, resistors, capacitors, etc.) by continual reductions in minimum feature size, which allow more components to be integrated into a given area. However, as the minimum features sizes are reduced, additional problems arise that should be addressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1A  to  FIG. 1C  illustrate a simplified perspective view, a circuit diagram, and a top down view of a memory structure in accordance with some embodiments of the disclosure. 
         FIG. 2  to  FIG. 40  illustrate various views of intermediate stages of manufacturing a memory structure in accordance with some embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated  90  degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     Various embodiments provide a semiconductor structure such as a 3D memory structure. In some embodiments, the 3D memory structure is a field effect transistor (FET) memory circuit including a plurality of vertically stacked memory cells. In some embodiments, each memory cell of the 3D memory structure is regarded as a FET that includes a word line region acting as a gate electrode, a bit line region acting as a first source/drain electrode, and a source line region acting as a second source/drain electrode, a gate dielectric, and an oxide semiconductor (OS) as a channel region. In some embodiments, each memory cell is regarded as a thin film transistor (TFT). 
     Conventionally, a plurality of pairs of source and drain regions of TFT in one column is aligned with another plurality of pairs of source and drain regions of TFT in an adjacent column in the active array of the 3D memory structure. With such aligned (or non-staggered) layout, the source line array and the bit line array connected above are usually formed with a plurality of metal layers, whereby its fabrication would involve complicated process flows and extra photomasks. In some embodiments of the present disclosure, the arrangement of memory cells in the active array and the routing/connections of the source line array and the bit line array are adjusted to simplify the process flow, and to reduce fabrication costs. 
       FIGS. 1A, 1B, and 1C  illustrate examples of a memory structure (or memory array) according to some embodiments of the present disclosure.  FIG. 1A  illustrates an example of a portion of a simplified memory structure  200  in a partial three-dimensional view;  FIG. 1B  illustrates a circuit diagram of the memory structure  200 ; and  FIG. 1C  illustrates a top down view of the memory structure  200  in accordance with some embodiments. The memory structure  200  includes a plurality of memory cells  202 , which may be arranged in a grid of rows and columns. The memory cells  202  may be further stacked vertically to provide a three-dimensional memory device, thereby increasing device density. For example, in some embodiments, the memory cells  202  are vertically stacked to form first sets of stacked memory cells  202 A arranged in one column, and are vertically stacked to form second sets of stacked memory cells  202 B arranged in another column. The first sets of stacked memory cells  202 A and second sets of stacked memory cells  202 B are arranged to be staggered from one another. Furthermore, the memory structure  200  may be disposed in the back end of line (BEOL) of a semiconductor die. For example, the memory device may be disposed in the interconnect layers of the semiconductor die, such as, above one or more active devices (e.g., transistors) formed on a semiconductor substrate. 
     Referring to  FIGS. 1A to 1C , in some embodiments, the memory structure  200  is a flash memory device, such as a NOR flash memory device, or the like. In some other embodiments, the memory structure  200  is another type of non-volatile memory array, such as a magnetoresistive random-access memory (MRAM) array, a resistive random-access memory (RRAM) array, or the like. In some embodiments, a gate of each memory cell  202  is electrically coupled to a respective word line (e.g., conductive line  72  (or conductive layer)), a first source/drain region of each memory cell  202  is electrically coupled to a respective bit line (e.g., bit line conductive layer BLC), and a second source/drain region of each memory cell  202  is electrically coupled to a respective source line (e.g., source line conductive layer SLC), which electrically couples the second source/drain region to ground. The memory cells  202  in a same horizontal row of the memory structure  200  may share a common word line while the memory cells  202  in a same vertical column of the memory structure  200  may share a common source line and a common bit line. 
     The memory structure  200  includes a plurality of vertically stacked conductive lines  72  (e.g., word lines) with dielectric layers  52  disposed between adjacent ones of the conductive lines  72 . The conductive lines  72  extend in a direction parallel to a major surface of an underlying substrate (not explicitly illustrated in  FIGS. 1A and 1B ). The alternately stacked conductive lines  72  (word line) and dielectric layers  52  may constitute a word line stack  58  of the memory structure. In some embodiments, the conductive lines  72  have a staircase configuration such that lower conductive lines  72  are longer than and extend laterally past endpoints of upper conductive lines  72 . For example, in  FIG. 1A , multiple, stacked layers of conductive lines  72  are illustrated with topmost conductive lines  72  being the shortest and bottommost conductive lines  72  being the longest. Respective lengths of the conductive lines  72  may increase in a direction towards the underlying substrate. In this manner, a portion of each of the conductive lines  72  may be accessible from above the memory structure  200 , and conductive contacts may be made to exposed portions of the conductive lines  72 , respectively. 
     The memory structure  200  further includes conductive pillars  106  (first semiconductor regions) electrically connected to bit lines, and conductive pillars  108  (second semiconductor regions) electrically connected to source lines, whereby the conductive pillars  106  and  108  are arranged alternately in the same column. The conductive pillars  106  and  108  (or electrode layers) may each extend in a direction perpendicular to the conductive lines  72 . A dielectric material  98 A/ 98 B is disposed between and isolates adjacent ones of the conductive pillars  106  and the conductive pillars  108 . Pairs of the conductive pillars  106  and  108  along with an intersecting conductive line  72  define boundaries of each memory cell  202 , and an isolation pillar  102  is disposed between and isolates adjacent pairs of the conductive pillars  106  and  108 . In some embodiments, the conductive pillars  108  are electrically coupled to ground. 
     The memory structure  200  may also include an oxide semiconductor (OS) material as a channel material layer  92 . The channel material layer  92  (or oxide semiconductor layer) may provide channel regions for the memory cells  202 . For example, when an appropriate voltage (e.g., higher than a respective threshold voltage (Vth) of a corresponding memory cell  202 ) is applied through a corresponding conductive line  72 , a region of the channel material layer  92  that intersects the conductive line  72  may allow current to flow from the conductive pillars  106  to the conductive pillars  108  (e.g., in the direction indicated by arrow  206 ). 
     In some embodiments, a dielectric layer  90  (or ferroelectric layer  90 ) is disposed between the channel material layer  92  and each of the conductive lines  72  and the dielectric layers  52 , and the dielectric layer  90  may serve as a gate dielectric for each memory cell  202 . In some embodiments, the dielectric layer  90  includes a ferroelectric material, such as a hafnium oxide, hafnium zirconium oxide, silicon-doped hafnium oxide, or the like. In some embodiments, the dielectric layer  90  includes a layer of SiNx between two SiOx layers (e.g., an ONO structure). 
     In some embodiments, when the dielectric layer  90  includes a ferroelectric material, the dielectric layer  90  may be polarized in one of two different directions, and the polarization direction may be changed by applying an appropriate voltage differential across the dielectric layer  90  and generating an appropriate electric field. The polarization may be relatively localized (e.g., generally contained within each boundaries of the memory cells  202 ), and a continuous region of the dielectric layer  90  may extend across a plurality of memory cells  202 . Depending on a polarization direction of a particular region of the dielectric layer  90  (or ferroelectric layer  90 ), a threshold voltage of a corresponding memory cell  202  varies, and a digital value (e.g., 0 or 1) can be stored. For example, when a region of the dielectric layer  90  (or ferroelectric layer  90 ) has a first electrical polarization direction, the corresponding memory cell  202  may have a relatively low threshold voltage, and when the region of the dielectric layer  90  has a second electrical polarization direction, the corresponding memory cell  202  may have a relatively high threshold voltage. The difference between the two threshold voltages may be referred to as the threshold voltage shift. A larger threshold voltage shift makes it easier (e.g., less error prone) to read the digital value stored in the corresponding memory cell  202 . 
     To perform a write operation on a memory cell  202 , a write voltage is applied across a portion of the dielectric layer  90  (or ferroelectric layer  90 ) corresponding to the memory cell  202 . In some embodiments, the write voltage is applied, for example, by applying appropriate voltages to a corresponding conductive line  72  (e.g., the word line) and the corresponding conductive pillars  106 / 108  (e.g., the first semiconductor region/the second semiconductor region). By applying the write voltage across the portion of the dielectric layer  90 , a polarization direction of the region of the dielectric layer  90  can be changed. As a result, the corresponding threshold voltage of the corresponding memory cell  202  can also be switched from a low threshold voltage to a high threshold voltage or vice versa, and a digital value can be stored in the memory cell  202 . Because the conductive lines  72  intersect the conductive pillars  106  and  108 , individual memory cells  202  may be selected for the write operation. 
     To perform a read operation on the memory cell  202  in such embodiments, a read voltage (a voltage between the low and high threshold voltages) is applied to the corresponding conductive line  72  (e.g., the world line). Depending on the polarization direction of the corresponding region of the dielectric layer  90  (or ferroelectric layer  90 ), the memory cell  202  may or may not be turned on. As a result, the conductive pillar  106  may or may not be discharged through the conductive pillar  108  (e.g., a source line that is coupled to ground), and the digital value stored in the memory cell  202  can be determined. As the conductive lines  72  intersect the conductive pillars  106  and  108 , individual memory cells  202  may be selected for the read operation. 
       FIG. 1A  further illustrates reference cross-sections of the memory structure  200  that are used in later figures. Cross-section B-B′ is along a longitudinal axis of conductive lines  72  and in a direction, for example, parallel to the direction of current flow of the memory cells  202 . Cross-section C-C′ is perpendicular to cross-section B-B′ and extends through the dielectric materials  98 A/ 98 B and the isolation pillars  102 . Cross-section D-D′ is perpendicular to cross-section B-B′ and extends through the dielectric materials  98 A/ 98 B and the conductive pillars  106 . Cross-section E-E′ is perpendicular to cross-section B-B′ and extends through the dielectric materials  98 A/ 98 B and the conductive pillars  108 . Subsequent figures refer to these reference cross-sections for clarity. 
       FIG. 2  to  FIG. 40  illustrate various views of intermediate stages of manufacturing a memory structure in accordance with some embodiments of the disclosure. Referring to  FIG. 2 , the fabrication of a CMOS (complementary metal oxide semiconductor) circuitry CX 1  is described. As illustrated in  FIG. 2 , a substrate  50  is provided. The substrate  50  may be a semiconductor substrate, such as a bulk semiconductor, a semiconductor-on-insulator (SOI) substrate, or the like, which may be doped (e.g., with a p-type or an n-type dopant) or undoped. The substrate  50  may be an integrated circuit die, such as a logic die, a memory die, an ASIC die, or the like. The substrate  50  may be a complementary metal oxide semiconductor (CMOS) die and may be referred to as a CMOS under array (CUA). The substrate  50  may be a wafer, such as a silicon wafer. Generally, an SOI substrate is a layer of a semiconductor material formed on an insulator layer. The insulator layer may be, for example, a buried oxide (BOX) layer, a silicon oxide layer, or the like. The insulator layer is provided on a substrate, typically a silicon or glass substrate. Other substrates, such as a multi-layered or gradient substrate may also be used. In some embodiments, the semiconductor material of the substrate  50  may include silicon; germanium; a compound semiconductor including silicon carbide, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including silicon-germanium, gallium arsenide phosphide, aluminum indium arsenide, aluminum gallium arsenide, gallium indium arsenide, gallium indium phosphide, and/or gallium indium arsenide phosphide; or combinations thereof. 
       FIG. 2  further illustrates circuits (part of the CMOS circuitry CX 1 ) that may be formed over the substrate  50 . The circuits include transistors at a top surface of the substrate  50 . The transistors may include gate dielectric layers  302  over top surfaces of the substrate  50  and gate electrodes  304  over the gate dielectric layers  302 . Source/drain regions  306  are disposed in the substrate  50  on opposite sides of the gate dielectric layers  302  and the gate electrodes  304 . Gate spacers  308  are formed along sidewalls of the gate dielectric layers  302  and separate the source/drain regions  306  from the gate electrodes  304  by appropriate lateral distances. The transistors may include fin field effect transistors (FinFETs), nanostructure (e.g., nanosheet, nanowire, gate-all-around, or the like) FETS (nano-FETs), planar FETs, the like, or combinations thereof, and may be formed by gate-first processes or gate-last processes. 
     A first inter-layer dielectric (ILD)  310  surrounds and isolates the source/drain regions  306 , the gate dielectric layers  302 , and the gate electrodes  304  and a second ILD  312  is located over the first ILD  310 . Source/drain contacts  314  extend through the second ILD  312  and the first ILD  310  and are electrically coupled to the source/drain regions  306 . Gate contacts  316  extend through the second ILD  312  and are electrically coupled to the gate electrodes  304 . An interconnect structure  320  is formed over the second ILD  312 , the source/drain contacts  314 , and the gate contacts  316 . The interconnect structure  320  includes one or more stacked dielectric layers  324  and conductive features  322  formed in the one or more dielectric layers  324 , for example. The interconnect structure  320  may be electrically connected to the gate contacts  316  and the source/drain contacts  314  to form functional circuits. In some embodiments, the functional circuits formed by the interconnect structure  320  may include logic circuits, memory circuits, sense amplifiers, controllers, input/output circuits, image sensor circuits, the like, or combinations thereof. Although  FIG. 2  discusses transistors formed over the substrate  50 , other active devices (e.g., diodes or the like) and/or passive devices (e.g., capacitors, resistors, or the like) may also be formed as part of the functional circuits. 
     Referring to  FIG. 3A  to  FIG. 8B , the fabrication processes of a bit line array AX 1  (illustrated in  FIG. 8A ) over the CMOS circuitry CX 1  is then described. As illustrated in  FIG. 3A , a first dielectric film DF 1  is formed over the substrate  50  and on the interconnect structure  320 . The first dielectric film DF 1  may be deposited by any suitable method, such as chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PEVCD), flowable CVD (FCVD), or the like. In some embodiments, the first dielectric film DF 1  may be formed of materials such as phospho-silicate glass (PSG), boro-silicate glass (BSG), boron-doped phospho-silicate glass (BPSG), undoped silicate glass (USG), or the like. In some embodiments, the first dielectric film DF 1  may include an oxide (e.g., silicon oxide or the like), a nitride (e.g., silicon nitride or the like), a combination thereof or the like. Other dielectric materials formed by any acceptable process may be used. 
     After forming the first dielectric film DF 1 , a first mask MK 1  is formed thereon. For example, from a top view of the first mask MK 1  illustrated in  FIG. 3B , the first mask MK 1  includes a plurality of stripe patterns MK 1 -SP spaced apart from one another. The stripe patterns MK 1 -SP are arranged on the first dielectric film DF 1  so that a portion of the first dielectric film DF 1  is revealed or exposed. In some alternative embodiments, the first mask MK 1  include other patterns, which may be adjusted based on the routing designs of the bit line array AX 1 . In some embodiments, the first mask MK 1  includes a positive photo-resist or a negative photo-resist. 
     Referring to  FIG. 4 , in a subsequent step, the first dielectric film DF 1  is patterned using the first mask MK 1  to form first openings OP 1  in the first dielectric film DF 1 . For example, portions of the first dielectric film DF 1  not covered by the first mask MK 1  are removed through acceptable etching process. Thereafter, referring to  FIG. 5A  and  FIG. 5B , a plurality of bit line conductive layers BLC is formed in the first openings OP 1  to fill up the first openings OP 1 . For example, the bit line conductive layers BLC are formed to be parallel to one another ( FIG. 5B ), but the disclosure is not limited thereto. In some embodiments, the bit line conductive layers BLC are electrically connected to the CMOS circuitry CX 1  below (e.g. connected to conductive features  322 ). In some embodiments, after the bit line conductive layers BLC are deposited, a planarization process, such as a chemical mechanical polish (CMP), an etch-back process, combinations thereof, or the like may be performed. The planarization process is performed so that top surfaces of the bit line conductive layers BLC are planar with a top surface of the first dielectric film DF 1 . 
     Referring to  FIG. 6A , after forming the bit line conductive layers BLC, a second dielectric film DF 2  is formed over the first dielectric film DF 1  and over the bit line conductive layers BLC. The second dielectric film DF 2  may be deposited by any suitable method, such as chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PEVCD), flowable CVD (FCVD), or the like. In some embodiments, the second dielectric film DF 2  may be formed of materials such as phospho-silicate glass (PSG), boro-silicate glass (BSG), boron-doped phospho-silicate glass (BPSG), undoped silicate glass (USG), or the like. In some embodiments, the second dielectric film DF 2  may include an oxide (e.g., silicon oxide or the like), a nitride (e.g., silicon nitride or the like), a combination thereof or the like. Other dielectric materials formed by any acceptable process may be used. The second dielectric film DF 2  and the first dielectric film DF 1  may include the same or different materials. 
     After forming the second dielectric film DF 3 , a second mask MK 2  is formed thereon. For example, from a top view of the second mask MK 2  illustrated in  FIG. 6B , the second mask MK 2  includes a plurality of via openings MK 2 -OP. The second mask MK 2  is arranged on the second dielectric film DF 2  so that the via openings MK 2 -OP reveals a portion of the first dielectric film DF 1 . In some embodiments, the second mask MK 2  includes a positive photo-resist or a negative photo-resist. 
     Referring to  FIG. 7 , in a subsequent step, the second dielectric film DF 2  is patterned using the second mask MK 2  to form second openings OP 2  in the second dielectric film DF 2 . For example, portions of the second dielectric film DF 2  not covered by the second mask MK 2  are removed through acceptable etching process. Thereafter, referring to  FIG. 8A  and  FIG. 8B , a first set of vias BLV 1  and a second set of vias BLV 2  (collectively referred as bit line vias) are formed in the second openings OP 2  to fill up the second openings OP 2 . 
     As illustrated in  FIG. 8B , the first set of vias BLV 1  are arranged on the bit line conductive layers BLC and spaced apart from one another along a second direction D 2 . Furthermore, the first set of vias BLV 1  may be arranged in columns and spaced apart from one another along a first direction D 1 , wherein the first direction D 1  is perpendicular to the second direction D 2 . In a similar way, the second set of vias BLV 2  are arranged on the bit line conductive layers BLC and spaced apart from one another along the second direction D 2 . Furthermore, the second set of vias BLV 2  may be arranged in columns and spaced apart from one another along the first direction D 1 . In the illustrated embodiment, the first set of vias BLV 1  are staggered widthwise in the second direction DL 2  from the second set of vias BLV 2 . In other words, the first set of vias BLV 1  and the second set of vias BLV 2  do not overlap one another in the second direction D 2 , or the first set of vias BLV 1  are misaligned with the second set of vias BLV 2 . The first set of vias BLV 1  and the second set of vias BLV 2  may be electrically connected to the bit line conductive layers BLC located underneath. However, in some embodiments, the first set of vias BLV 1  and the second set of vias BLV 2  are not located on the same stripe of bit line conductive layers BLC. Up to here, a bit line array AX 1  is formed over the CMOS circuitry CX 1 , and the bit line array AX 1  is electrically connected to the interconnect structure  320 . 
     Referring to  FIGS. 9A and 9B , a multi-layer stack  58  (or word line stack) is formed over the structure of  FIG. 8A . The substrate  50 , the transistors, the ILDs, the interconnect structure  320 , and the bit line array AX 1  may be omitted from subsequent drawings for the purposes of simplicity and clarity. Although the bit line array AX 1  and the interconnect structure  320  are illustrated as being located in between the multi-layer stack  58  and the substrate  50  and contacting one another, it is noted that any number of intermediate layers may be further disposed between the substrate  50  and the multi-layer stack  58  (or word line stack). For example, one or more additional interconnect layers comprising conductive features in insulting layers (e.g., low-k dielectric layers) may be disposed between the substrate  50  and the multi-layer stack  58 . In some embodiments, the conductive features may be patterned to provide power, ground, and/or signal lines for the active devices on the substrate  50  and/or the memory structure  200  (see  FIGS. 1A and 1B ). In some embodiments, one or more interconnect layers including conductive features in insulting layers (e.g., low-k dielectric layers) may be disposed over the multi-layer stack  58  (or word line stack). 
     As illustrated in  FIGS. 9A and 9B , the multi-layer stack  58  (or word line stack) includes alternating layers of sacrificial layers  53 A- 53 D (collectively referred to as sacrificial layers  53 ) and dielectric layers  52 A- 52 E (collectively referred to as dielectric layers  52 ). The sacrificial layers  53  may be patterned and replaced in subsequent steps to define conductive lines  72  (e.g., the word lines). The sacrificial layers  53  may include dielectric materials, such as silicon oxide, silicon nitride, silicon oxynitride, combinations thereof, or the like. The dielectric layers  52  may include insulating materials, such as silicon oxide, silicon nitride, silicon oxynitride, combinations thereof, or the like. The sacrificial layers  53  and the dielectric layers  52  include different materials with different etching selectivities. In some embodiments, the sacrificial layers  53  include silicon nitride, and the dielectric layers  52  include silicon oxide. Each of the sacrificial layers  53  and the dielectric layers  52  may be formed using, for example, chemical vapor deposition (CVD), atomic layer deposition (ALD), physical vapor deposition (PVD), plasma enhanced CVD (PECVD), or the like. 
     Although  FIGS. 9A and 9B  illustrates a particular number of the sacrificial layers  53  and the dielectric layers  52 , other embodiments may include different numbers of the sacrificial layers  53  and the dielectric layers  52 . Besides, although the multi-layer stack  58  is illustrated as having dielectric layers as topmost and bottommost layers, the disclosure is not limited thereto. In some embodiments, at least one of the topmost and bottommost layers of the multi-layer stack  58  is a sacrificial layer. 
       FIG. 10  through  FIG. 17  and  FIG. 18B  are various views of intermediate stages in the manufacturing a staircase structure of the memory structure  200 , in accordance with some embodiments.  FIGS. 10  through  FIG. 17  and  FIG. 18B  are illustrated along reference cross-section B-B′ illustrated in  FIG. 1A .  FIG. 18A  is illustrated as a three-dimensional view of the structure shown in  FIG. 18B . 
     As illustrated in  FIG. 10 , a photoresist  56  is formed over the multi-layer stack  58 . In some embodiments, the photoresist  56  is formed by a spin-on technique and patterned by an acceptable photolithography technique. Patterning the photoresist  56  may expose the multi-layer stack  58  in regions  60 , while masking remaining portions of the multi-layer stack  58 . For example, a topmost layer of the multi-layer stack  58  (e.g., the dielectric layer  52 E) may be exposed in the regions  60 . 
     As illustrated in  FIG. 11 , the exposed portions of the multi-layer stack  58  in the regions  60  are etched using the photoresist  56  as a mask. The etching may be any acceptable etching process, such as a dry etch (e.g., a reactive ion etch (RIE), a neutral beam etch (NBE), the like), a wet etch, the like, or a combination thereof. The etching may be anisotropic. The etching may remove portions of the dielectric layer  52 E and the sacrificial layer  53 D in the regions  60  and define openings  61 . Because the dielectric layer  52 E and the sacrificial layer  53 D have different material compositions, etchants used to remove exposed portions of these layers may be different. In some embodiments, the sacrificial layer  53 D acts as an etch stop layer while etching the dielectric layer  52 E, and the dielectric layer  52 D acts as an etch stop layer while etching sacrificial layer  53 D. As a result, the portions of the dielectric layer  52 E and the sacrificial layer  53 D may be selectively removed without removing remaining layers of the multi-layer stack  58 , and the openings  61  may be extended to a desired depth. Alternatively, a time-mode etching process may be used to stop the etching of the openings  61  after the openings  61  reach a desired depth. In the resulting structure, the dielectric layer  52 D is exposed in the regions  60 . 
     As illustrated in  FIG. 12 , the photoresist  56  is trimmed to expose additional portions of the multi-layer stack  58 . In some embodiments, the photoresist  56  is trimmed by using an acceptable removing technique such as a lateral etching. As a result of the trimming, a width of the photoresist  56  is reduced and portions the multi-layer stack  58  in the regions  60  and regions  62  may be exposed. For example, top surfaces of the dielectric layer  52 D may be exposed in the regions  60 , and top surfaces of the dielectric layer  52 E may be exposed in the regions  62 . 
     As illustrated in  FIG. 13 , portions of the dielectric layer  52 E, the sacrificial layer  53 D, the dielectric layer  52 D, and the sacrificial layer  53 C in the regions  60  and the regions  62  are removed by acceptable etching processes using the photoresist  56  as a mask. The etching may be any acceptable etching process, such as a dry etch (e.g., RIE, NBE, the like), a wet etch, the like, or a combination thereof. The etching may be anisotropic. The etching may extend the openings  61  further into the multi-layer stack  58 . Because the sacrificial layers  53 D and  53 C and the dielectric layers  52 E and  52 D have different material compositions, etchants used to remove exposed portions of these layers may be different. In some embodiments, portions of the dielectric layers  52 E and  52 D in the regions  62  and  60  are removed by using the photoresist  56  as a mask and using the underlying sacrificial layers  53 D and  53 C as etch stop layers. Thereafter, the exposed portions of the sacrificial layers  53 D and  53 C in the regions  62  and  60  are removed by using the photoresist  56  as a mask and using the underlying dielectric layers  52 D and  52 C as etching stop layers. In the resulting structure, the dielectric layer  52 C is exposed in the regions  60 , and the dielectric layer  52 D is exposed in the regions  62 . 
     As illustrated in  FIG. 14 , the photoresist  56  is trimmed to expose additional portions of the multi-layer stack  58 . In some embodiments, the photoresist  56  is trimmed by using an acceptable removing technique such as a lateral etching. As a result of the trimming, a width of the photoresist  56  is reduced, and portions the multi-layer stack  58  in the regions  60 , the regions  62 , and regions  64  may be exposed. For example, top surfaces of the dielectric layer  52 C may be exposed in the regions  60 ; top surfaces of the dielectric layer  52 D may be exposed in the regions  62 ; and top surfaces of the dielectric layer  52 E may be exposed in the regions  64 . 
     As illustrated in  FIG. 15 , portions of the dielectric layers  52 E,  52 D, and  52 C and the sacrificial layers  53 D,  53 C, and  53 B in the regions  60 , the regions  62 , and the regions  64  are removed by acceptable etching processes using the photoresist  56  as a mask. The etching may be any acceptable etching process, such as a dry etch (e.g., RIE, NBE, the like), a wet etch, the like, or a combination thereof. The etching may be anisotropic. The etching may extend the openings  61  further into the multi-layer stack  58 . Because the dielectric layers  52 C- 52 E and the sacrificial layers  53 B- 53 D have different material compositions, etchants used to remove exposed portions of these layers may be different. In some embodiments, portions of the dielectric layers  52 E,  52 D and  52 C in the regions  64 ,  62  and  60  are removed by using the photoresist  56  as a mask and using the underlying sacrificial layers  53 D,  53 C and  53 B as etch stop layers. Thereafter, the exposed portions of the sacrificial layers  53 D,  53 C and  53 B in the regions  64 ,  62  and  60  are removed by using the photoresist  56  as a mask and using the underlying dielectric layers  52 D,  52 C and  52 B as etching stop layers. In the resulting structure, the dielectric layer  52 B is exposed in the regions  60 ; the dielectric layer  52 C is exposed in the regions  62 ; and the dielectric layer  52 D is exposed in the regions  64 . 
     As illustrated in  FIG. 16 , the photoresist  56  is trimmed to expose additional portions of the multi-layer stack  58 . In some embodiments, the photoresist  56  is trimmed by using an acceptable removing technique such as a lateral etching. As a result of the trimming, a width of the photoresist  56  is reduced, and portions the multi-layer stack  58  in the regions  60 , the regions  62 , the regions  64 , and regions  66  may be exposed. For example, top surfaces of the dielectric layer  52 B may be exposed in the regions  60 ; top surfaces of the dielectric layer  52 C may be exposed in the regions  62 ; and top surfaces of the dielectric layer  52 D may be exposed in the regions  64 ; and top surfaces of the dielectric layer  52 E may be exposed in the regions  66 . 
     As illustrated in  FIG. 17 , portions of the dielectric layers  52 E,  52 D,  52 C, and  52 B in the regions  60 , the regions  62 , the regions  64 , and the regions  66  are removed by acceptable etching processes using the photoresist  56  as a mask. The etching may be any acceptable etching process, such as a dry etch (e.g., RIE, NBE, the like), a wet etch, the like, or a combination thereof. The etching may be anisotropic. The etching may extend the openings  61  further into the multi-layer stack  58 . In some embodiments, portions of the dielectric layers  52 E,  52 D,  52 C and  52 B in the regions  66 ,  64 ,  62  and  60  are removed by using the photoresist  56  as a mask and using the underlying sacrificial layers  53 D,  53 C,  53 B and  53 A as etch stop layers. In the resulting structure, the sacrificial layer  53 A is exposed in the regions  60 ; the sacrificial layer  53 B is exposed in the regions  62 ; the sacrificial layer  53 C is exposed in the regions  64 ; and the sacrificial layer  53 D is exposed in the regions  66 . Thereafter, the photoresist  56  may be removed by an acceptable ashing or wet strip process. 
     As illustrated in  FIGS. 18A and 18B , an inter-metal dielectric (IMD)  70  is deposited over the multi-layer stack  58 . The IMD  70  may be formed of a dielectric material, and may be deposited by any suitable method, such as CVD, PECVD, flowable CVD (FCVD), or the like. The dielectric materials may include phospho-silicate glass (PSG), boro-silicate glass (BSG), boron-doped phospho-silicate glass (BPSG), undoped silicate glass (USG), or the like. In some embodiments, the IMD  70  may include an oxide (e.g., silicon oxide or the like), a nitride (e.g., silicon nitride or the like), a combination thereof or the like. Other dielectric materials formed by any acceptable process may be used. The IMD  70  extends along sidewalls of the sacrificial layers  53 B- 53 D and sidewalls of the dielectric layers  52 B- 52 E. Further, the IMD  70  may contact top surfaces of the sacrificial layers  53 A- 53 D and the dielectric layer  52 E. 
     Thereafter, a removal process is applied to the IMD  70  to remove excess dielectric material over the multi-layer stack  58 . In some embodiments, the removal process may be a planarization process, such as a chemical mechanical polish (CMP), an etch-back process, combinations thereof, or the like. The planarization process exposes the multi-layer stack  58  such that top surfaces of the multi-layer stack  58  and IMD  70  are leveled after the planarization process is completed. 
     As shown in  FIGS. 18A and 18B , an intermediate and bulk staircase structure is thus formed. The intermediate staircase structure includes alternating layers of sacrificial layers  53  and dielectric layers  52 . The sacrificial layers  53  are subsequently replaced with conductive lines  72 , which will be described in details in  FIGS. 22A and 22B . Lower conductive lines  72  are longer and extend laterally past upper conductive lines  72 , and a width of each of the conductive lines  72  increases in a direction towards the substrate  50  (see  FIG. 1A ). 
       FIG. 19  through  FIG. 22B  are various views of intermediate stages in the manufacturing of a memory region of the memory structure  200 , in accordance with some embodiments of the disclosure. In  FIG. 19  through  FIG. 22B , the bulk multi-layer stack  58  (word line stack) is patterned to form trenches  86  therethrough, and sacrificial layers  53  are replaced with conductive materials to define the conductive lines  72 . The conductive lines  72  may correspond to word lines in the memory structure  200 , and the conductive lines  72  may further provide gate electrodes for the resulting memory cells of the memory structure  200 .  FIGS. 19, 20, 21B and 22B  are illustrated along reference cross-section C-C′ illustrated in  FIG. 1A .  FIGS. 21A and 22A  are illustrated in a partial three-dimensional view. 
     As illustrated in  FIG. 19 , photoresist patterns  82  and underlying hard mask patterns  80  are formed over the multi-layer stack  58 . In some embodiments, a hard mask layer and a photoresist layer are sequentially formed over the multi-layer stack  58 . The hard mask layer may include, for example, silicon nitride, silicon oxynitride, or the like, which may be deposited by CVD, PVD, ALD, PECVD, or the like. The photoresist layer is formed by a spin-on technique, for example. 
     Thereafter, the photoresist layer is patterned to form photoresist patterns  82  and trenches  86  between the photoresist patterns  82 . The photoresist layer is patterned by an acceptable photolithography technique, for example. The patterns of the photoresist patterns  82  are then transferred to the hard mask layer to form hard mask patterns  80  by using an acceptable etching process, such as by a dry etch (e.g., RIE, NBE, the like), a wet etch, the like, or a combination thereof. The etching may be anisotropic. Thus, trenches  86  are formed extending through the hard mask layer. Thereafter, the photoresist  82  may be optionally removed by an ashing process, for example. 
     As illustrated in  FIGS. 20 to 21B , the patterns of the hard mask patterns  80  are transferred to the multi-layer stack  58  using one or more acceptable etching processes, such as by a dry etch (e.g., RIE, NBE, the like), a wet etch, the like, or a combination thereof. The etching processes may be anisotropic. Thus, the trenches  86  extend through the bulk multi-layer stack  58 , and strip-shaped sacrificial layers  53  and strip-shaped dielectric layers  52  are accordingly defined. In some embodiments, the trenches  86  extend through the bulk staircase structure, and strip-shaped staircase structures are accordingly defined. The hard mask patterns  80  may be then removed by an acceptable process, such as a wet etching process, a dry etching process, a planarization process, combinations thereof, or the like. 
     As illustrated in  FIGS. 22A  and  FIG. 22B , the sacrificial layers  53 A- 53 D (collectively referred to as sacrificial layers  53 ) are replaced with conductive lines  72 A- 72 D (collectively referred to as conductive lines  72 ). In some embodiments, the sacrificial layers  53  are removed by an acceptable process, such as a wet etching process, a dry etching process or both. Thereafter, conductive lines  72  are filled into the space between two adjacent dielectric layers  52 . As shown in the local enlarged view, each conductive line  72  includes two barrier layers  71  and  75  and a metal layer  73  between the barrier layers  71  and  75 . In some embodiments, a barrier layer is disposed between the metal layer  73  and the adjacent dielectric layer  52 . The barrier layers may prevent the metal layer from diffusion to the adjacent dielectric layers  52 . The barrier layers may also provide the function of increasing the adhesion between the metal layer and the adjacent dielectric layers, and may be referred to as glue layers in some examples. In some embodiments, both barrier layers and glue layers with different materials are provided as needed. The barrier layers  71  and  75  are formed of a first conductive material, such as a metal nitride, such as titanium nitride, tantalum nitride, molybdenum nitride, zirconium nitride, hafnium nitride, or the like. The metal layer  73  may are formed of a second conductive material, such as a metal, such as tungsten, ruthenium, molybdenum, cobalt, aluminum, nickel, copper, silver, gold, alloys thereof, or the like. The barrier layers  71 ,  75  and metal layer  73  may each be formed by an acceptable deposition process such as CVD, PVD, ALD, PECVD, or the like. The barrier layers  71 ,  75  and the metal layer  73  are further deposited on the sidewalls of the multi-layer stack  58  (word line stack) and fill in the trenches  86 . Thereafter, the barrier layers  71 ,  75  and the metal layer  73  in the trenches  86  are removed by an etching back process. An acceptable etch back process may be performed to remove excess materials from the sidewalls of the dielectric layers  52  and the bottom surfaces of the trenches  86 . The acceptable etch back process includes a dry etch (e.g., RIE, NBE, the like), a wet etch, the like, or a combination thereof. The acceptable etch back process may be anisotropic. 
     In some embodiments, upon the replacement process, the sacrificial layers  53  of the strip-shaped staircase structures are subsequently replaced with conductive lines  72  (see  FIG. 1A ) in the multi-layer stack  58  (word line stack). 
       FIG. 23A  through  FIG. 28B  illustrate forming and patterning channel regions for the memory cells  202  (see  FIG. 1A ) in the trenches  86 .  FIGS. 23A, 24A and 28A  are illustrated in a partial three-dimensional view. In  FIGS. 23B, 24B, 25, 26, 227 and 28B  cross-sectional views are provided along line C-C′ of  FIG. 1A . As illustrated in  FIG. 23A  through  FIG. 26 , a dielectric layer  90  (ferroelectric layer), a channel material layer  92  (oxide semiconductor layer), and a dielectric material  98 A are deposited in the trenches  86 . 
     As illustrated in  FIG. 23A  and  FIG. 23B , a dielectric layer  90  (or ferroelectric layer) may be deposited conformally in the trenches  86  along sidewalls of the conductive lines  72 , sidewalls of the dielectric layers  52 , over top surfaces of the dielectric layer  52 E and along the bottom surfaces of the trenches  86 . In some embodiments, a dielectric layer  90  (or ferroelectric layer) may be further deposited on the IMD  70  and along the sidewall of each step of the staircase structure in the staircase region. The dielectric layer  90  (or ferroelectric layer) may include materials that are capable of switching between two different polarization directions by applying an appropriate voltage differential across the dielectric layer  90 . For example, the dielectric layer  90  includes a high-k dielectric material, such as a hafnium (Hf) based dielectric materials or the like. In some embodiments, the dielectric layer  90  includes hafnium oxide, hafnium zirconium oxide, silicon-doped hafnium oxide, or the like. 
     n some other embodiments, the dielectric layer  90  may include barium titanium oxide (BaTiO 3 ), lead titanium oxide (PbTiO 3 ), lead zirconium oxide (PbZrO 3 ), lithium niobium oxide (LiNbO 3 ), sodium niobium oxide (NaNbO 3 ), potassium niobium oxide (KNbO 3 ), potassium tantalum oxide (KTaO 3 ), bismuth scandium oxide (BiScO 3 ), bismuth iron oxide (BiFeO 3 ), hafnium erbium oxide (Hf 1-x Er x O), hafnium lanthanum oxide (Hf 1-x La x O), hafnium yttrium oxide (Hf 1-x Y x O), hafnium gadolinium oxide (Hf 1-x Gd x O), hafnium aluminum oxide (Hf 1-x Al x O), hafnium zirconium oxide (Hf 1-x Zr x O, HZO), hafnium titanium oxide (Hf 1-x Ti x O), hafnium tantalum oxide (Hf 1-x Ta x O), or the like. In some embodiments, the dielectric layer  90  may include different ferroelectric materials or different types of memory materials. For example, in some embodiments, the dielectric layer  90  is a non-ferroelectric material, such as a multilayer memory structure including a layer of SiN x  between two SiO x  layers (e.g., an ONO structure). In some embodiments, the method of forming the dielectric layer  90  includes performing a suitable deposition technique, such as CVD, PECVD, metal oxide chemical vapor deposition (MOCVD), ALD, remote plasma ALD (RPALD), plasma enhanced ALD (PEALD), molecular beam deposition (MBD) or the like. 
     In some embodiments, the dielectric layer  90  has a thickness of about 1-20 nm, such as 5-10 nm. Other thickness ranges (e.g., more than 20 nm or 5-15 nm) may be applicable. In some embodiments, the dielectric layer  90  is formed in a fully amorphous state. In alternative embodiments, the dielectric layer  90  is formed in a partially crystalline state; that is, the dielectric layer  90  is formed in a mixed crystalline-amorphous state and having some degree of structural order. In yet alternative embodiments, the dielectric layer  90  is formed in a fully crystalline state. In some embodiments, the dielectric layer  90  is a single layer. In alternative embodiments, the dielectric layer  90  is a multi-layer structure. 
     In some embodiments, an annealing process is performed to the dielectric layer  90 . In some embodiments, upon the annealing process, the dielectric layer  90  is transformed from an amorphous state to a partially or fully crystalline sate. In alternative embodiments, upon the annealing process, the dielectric layer  90  is transformed from a partially crystalline state to a fully crystalline sate. 
     As illustrated in  FIG. 24A  and  FIG. 24B , a channel material layer  92  is conformally deposited in the trenches  86  over the dielectric layer  90 . The channel material layer  92  includes materials suitable for providing channel regions for the memory cells  202  (see  FIG. 1A ). For example, the channel material layer  92  includes oxide semiconductor (OS) such as zinc oxide (ZnO), indium tungsten oxide (InWO), indium gallium zinc oxide (InGaZnO, IGZO), indium zinc oxide (InZnO), indium tin oxide (ITO), combinations thereof, or the like. The channel material layer  92  may be deposited by CVD, PVD, ALD, PECVD, or the like. The channel material layer  92  may extend along sidewalls and bottom surfaces of the trenches  86  over the dielectric layer  90 . In some embodiments, the channel material layer  92  may be further deposited on the IMD  70  and along the sidewall of each step of the staircase structure in the staircase region. 
     As illustrated in  FIG. 25 , in a subsequent step, a dielectric material  98 A is deposited in the trenches  86  over the channel material layer  92 . In some embodiments, the dielectric material  98 A includes silicon oxide, silicon nitride, silicon oxynitride, or the like, which may be deposited by CVD, PVD, ALD, PECVD, or the like. The dielectric material  98 A may extend along sidewalls and bottom surfaces of the trenches  86  over the channel material layer  92 . In some embodiments, the dielectric material  98 A is optional and may be omitted as needed. 
     As illustrated in  FIG. 26 , bottom portions of the dielectric material  98 A and the channel material layer  92  are removed in the trenches  86 . The removal process includes an acceptable etching process, such as a dry etch (e.g., RIE, NBE, the like), a wet etch, the like, or a combination thereof. The etching may be anisotropic. Accordingly, the remaining dielectric material  98 A and the channel material layer  92  may expose portions of the dielectric layer  90  on bottom surfaces of the trenches  86 . Thus, portions of the channel material layer  92  on opposing sidewalls of the trenches  86  may be separated from each other, which improves isolation between the memory cells  202  of the memory structure  200  (see  FIG. 1A ). 
     As illustrated in  FIG. 27 , a dielectric material  98 B is deposited to completely fill the trenches  86 . The dielectric material  98 B may be formed of one or more materials and by processes the same as or similar to those of the dielectric material  98 A. In some alternative embodiments, the dielectric material  98 B and the dielectric material  98 A include different materials. 
     As illustrate in  FIG. 28A  and  FIG. 28B , a removal process is applied to the dielectric materials  98 A/ 98 B, the channel material layer  92 , and the dielectric layer  90  to remove excess materials over the multi-layer stack  58  (word line stack). In some embodiments, a planarization process such as a CMP, an etch-back process, combinations thereof, or the like may be utilized. The planarization process exposes the multi-layer stack  58  (word line stack) such that top surfaces of the multi-layer stack  58  (e.g., the dielectric layer  52 E), the dielectric layer  90 , the channel material layer  92 , the dielectric materials  98 A/ 98 B, and the IMD  70  (in the staircase region) are leveled after the planarization process is completed. 
       FIG. 29A  through  FIG. 32C  illustrate intermediate steps of manufacturing conductive pillars  106  and  108  (source/drain regions, e.g. first semiconductor regions  106  and second semiconductor regions  108 ) in the memory structure  200 . The conductive pillars  106  and  108  (first and second semiconductor regions  106  and  108 ) may extend along a direction perpendicular to the conductive lines  72  such that individual cells of the memory structure  200  may be selected for read and write operations.  FIGS. 29A, 30A, 31A and 32A  are illustrated in a partial three-dimensional view. In  FIGS. 29B and 30B , cross-sectional views are provided along line C-C′ of  FIG. 1A . In  FIGS. 31B and 32B , cross-sectional views are provided along line D-D′ of  FIG. 1A . In  FIG. 32C , a cross-sectional view is provided along line E-E′ of  FIG. 1A . 
     As illustrated in  FIGS. 29A and 29B , trenches  100  are patterned through the channel material layer  92  and the dielectric materials  98 A/ 98 B. For example, the dielectric materials  98 A/ 98 B are patterned to form dielectric pillars separated by the trenches  100 . Patterning the trenches  100  may be performed through a combination of photolithography and etching, for example. The trenches  100  may be disposed between opposing sidewalls of the dielectric layer  90 , and the trenches  100  may physically separate adjacent stacks of memory cells in the memory structure  200  (see  FIG. 1A ). 
     As illustrated in  FIGS. 30A and 30B , isolation pillars  102  are formed in the trenches  100 . In some embodiments, an isolation layer is deposited over the multi-stack  58  filling in the trenches  100 . The isolation layer may include, for example, silicon oxide, silicon nitride, silicon oxynitride, or the like, which may be deposited by CVD, PVD, ALD, PECVD, or the like. The isolation layer may extend along sidewalls and bottom surfaces of the trenches  100  over the channel material layer  92 . After deposition, a planarization process (e.g., a CMP, etch back, or the like) may be performed to remove excess portions of the isolation layer to form the isolation pillars  102 . In the resulting structure, top surfaces of the multi-layer stack  58  (e.g., dielectric layer  52 E), the dielectric layer  90 , the channel material layer  92 , and the isolation pillars  102  may be substantially leveled (e.g., within process variations). In some embodiments, the isolation pillars  102  have a staggered arrangement. In some embodiments, materials of the dielectric materials  98 A/ 98 B and isolation pillars  102  may be selected so that they may be etched selectively relative to each other. For example, in some embodiments, the dielectric materials  98 A/ 98 B include oxide and the isolation pillars  102  include nitride. In some embodiments, the dielectric materials  98 A/ 98 B (or dielectric pillars) include nitride and the isolation pillars  102  include oxide. Other materials are also possible. 
     As illustrated in  FIG. 31A  and  FIG. 31B , trenches  104  are defined for the subsequently formed conductive pillars  106  and  108  (the first and second semiconductor regions  106  and  108 ). For example, the dielectric materials  98 A/ 98 B (or dielectric pillars) are further patterned to define the trenches  104 . The trenches  104  are formed by patterning the dielectric materials  98 A/ 98 B (or dielectric pillars) with a combination of photolithography and etching, for example. In some embodiments, as shown in  FIG. 31A , a photoresist  118  is formed over the multi-layer stack  58 , the dielectric materials  98 A/ 98 B, the isolation pillars  102 , the channel material layer  92 , and the dielectric layer  90 . In some embodiments, the photoresist  118  is patterned by an acceptable photolithography technique to define openings  120 . Each of the openings  120  may expose the corresponding isolation pillar  102  and two separate regions of the dielectric materials  98 A/ 98 B beside the isolation pillar  102 . In this way, each of the openings  120  may define a pattern of a conductive pillar  106  and an adjacent conductive pillar  108  that are separated by the isolation pillars  102 . 
     Subsequently, portions of the dielectric materials  98 A/ 98 B (or dielectric pillars) exposed by the openings  120  may be removed by an acceptable etching process, such as by a dry etch (e.g., RIE, NBE, the like), a wet etch, the like, or a combination thereof. The etching may be anisotropic. The etching process may use an etchant that etches the dielectric materials  98 A/ 98 B without significantly etching the isolation pillars  102 . As a result, even though the openings  120  expose the isolation pillars  102 , the isolation pillars  102  may not be significantly removed. Patterns of the trenches  104  may correspond to the conductive pillars  106  and  108  (see  FIG. 32A  to  FIG. 32C ). After the trenches  104  are patterned, the photoresist  118  may be removed by ashing, for example. 
     As illustrated in  FIG. 32A  to  FIG. 32C , the trenches  104  are filled with a conductive material to form the conductive pillars  106  and  108  (the first and second semiconductor regions  106  and  108 ). The conductive material may include copper, titanium, titanium nitride, tantalum, tantalum nitride, tungsten, ruthenium, aluminum, combinations thereof, or the like, which may be formed using, for example, CVD, ALD, PVD, PECVD, or the like. After the conductive material is deposited, a planarization (e.g., a CMP, etch back, or the like) may be performed to remove excess portions of the conductive material, thereby forming the conductive pillars  106  and  108 . In the resulting structure, top surfaces of the multi-layer stack  58  (e.g., the dielectric layer  52 E), the dielectric layer  90 , the channel material layer  92 , the conductive pillars  106 , and the conductive pillars  108  may be substantially leveled (e.g., within process variations). In some embodiments, the conductive pillars  106  (first semiconductor region  106 ) correspond to and are electrically connected to the bit line array AX 1  in the memory structure  200 , and the conductive pillars  108  (second semiconductor region  108 ) correspond to and are electrically connected to the source line array formed in subsequent steps. 
     Thus, stacked memory cells  202  may be formed in the memory structure  200 , as shown in  FIG. 32A . Each memory cell  202  includes a gate electrode (e.g., a portion of a corresponding conductive line  72 ), a gate dielectric (e.g., a portion of a corresponding dielectric layer  90 ), a channel region (e.g., a portion of a corresponding channel material layer  92 ), and source/drain pillars (e.g., portions of corresponding conductive pillars  106  and  108 ). The isolation pillars  102  isolates adjacent memory cells  202  in a same column and at a same vertical level. The memory cells  202  may be disposed in an array of vertically stacked rows and columns. 
     As illustrated in  FIG. 32A , in some embodiments, the formed memory structure  200  (or active array) includes a word line stack  58  having a first surface S 1  and a second surface S 2  opposite to the first surface S 1 . The conductive pillars  106  and  108  (first and second semiconductor regions) are alternately arranged along the first surface S 1  of the word line stack  58  and spaced apart from one another in the first direction D 1 . Similarly, the conductive pillars  106  and  108  (first and second semiconductor regions) are alternately arranged along the second surface S 2  of the word line stack  58 , and spaced apart from one another in the first direction D 1 . In some embodiments, the conductive pillars  106  and  108  (first and second semiconductor regions) on the first surface S 1  are arranged to be staggered widthwise in a second direction D 2  from the conductive pillars  106  and  108  (first and second semiconductor regions) on the second surface D 2 , wherein the second direction D 2  being perpendicular to the first direction D 1 . Furthermore, the bit line array AX 1  disposed below the word line stack  58  may be connected to the conductive pillars  106  (first semiconductor regions) on the first surface S 1  and the second surface S 2  of the word line stack  58 . For examples, the first set of vias BLV 1  are connected to the conductive pillars  106  (first semiconductor regions) on the first surface S 1  of the word line stack  58 , while the second set of vias BLV 2  are connected to the conductive pillars  106  (first semiconductor regions) on the second surface S 2  of the word line stack  58 . 
     As further illustrated in  FIG. 32A , in some embodiments, the memory cells  202  may be stacked up to form first sets of stacked memory cells  202 A and second sets of stacked memory cells  202 B. In some embodiments, the first sets of stacked memory cells  202 A are arranged along a first column CLM 1  in an active array of the memory structure  200 , and the second sets of stacked memory cells  202 B are arranged along a second column CLM 2  in an active array of the memory structure  200 . The second column CLM 2  being parallel to the first column CLM 1 . Each of the first sets of stacked memory cells  202 A includes conductive pillars  106  and  108  (first and second conductive pillars), a dielectric pillar (e.g., dielectric materials  98 A/ 98 B) disposed in between the conductive pillar  106  and the conductive pillar  108 , and channel material layers  92  located aside the dielectric pillar ( 98 A/ 98 B) and the conductive pillars  106  and  108 . Similarly, each of the second sets of stacked memory  202 B includes conductive pillars  106  and  108  (third and fourth conductive pillars), a dielectric pillar (e.g., dielectric materials  98 A/ 98 B) disposed in between the conductive pillar  106  and the conductive pillar  108 , and channel material layers  92  located aside the dielectric pillar ( 98 A/ 98 B) and the conductive pillars  106  and  108 . In some embodiments, the conductive pillars  106  and  108  (third and fourth conductive pillars) in the second sets of stacked memory cells  202 B are misaligned with the conductive pillars  106  and  108  (first and second conductive pillars) in the first sets of stacked memory cells  202 A. 
     Furthermore, in some embodiments, a word line stack  58  (including dielectric layers  52  and conductive lines  72 ) is arranged along a third column CLM 3  in the active array between the first column CLM 1  and the second column CLM 2 , and disposed in between the first sets of stacked memory cells  202 A and the second sets of stacked memory cells  202 B. In certain embodiments, the word line stack  58  is shared between the first sets of stacked memory cells  202 A and the second sets of stacked memory cells  202 B. 
     As illustrated in  FIG. 33A  to  FIG. 39C , a source line array AX 2  is formed on top surfaces of the multi-layer stack  58  (e.g., the dielectric layer  52 E), the dielectric layer  90 , the channel material layer  92 , the conductive pillars  106 , and the conductive pillars  108  and the IMD  70  (in the staircase region).  FIG. 33A ,  FIG. 34 ,  FIG. 35A ,  FIG. 36A ,  FIG. 37  and  FIG. 38A  illustrate cross-sectional views of the memory structure  200  along line E-E′ of  FIG. 1A .  FIG. 33B ,  FIG. 35B ,  FIG. 36B ,  FIG. 38B  illustrate top-down views of the memory structure  200  at various stages.  FIG. 39A  illustrates a perspective view of the memory structure  200 ;  FIG. 39B  illustrates a cross-sectional view of the memory structure  200  along line F-F′ of  FIG. 39A ;  FIG. 39C  illustrates a cross-sectional view of the memory structure  200  along line B-B′ of  FIG. 1A . 
     As illustrated in  FIG. 33A , a third dielectric film DF 3  is formed over the multi-layer stack  58  (word line stack) and over the conductive pillars  106  and  108 . The third dielectric film DF 3  may be deposited by any suitable method, such as CVD, PEVCD, FCVD, or the like. In some embodiments, the third dielectric film DF 3  may be formed of materials such as phospho-silicate glass (PSG), boro-silicate glass (BSG), boron-doped phospho-silicate glass (BPSG), undoped silicate glass (USG), or the like. In some embodiments, the third dielectric film DF 3  may include an oxide (e.g., silicon oxide or the like), a nitride (e.g., silicon nitride or the like), a combination thereof or the like. Other dielectric materials formed by any acceptable process may be used. 
     After forming the third dielectric film DF 3 , a third mask MK 3  is formed thereon. For example, from a top view of the third mask MK 3  illustrated in  FIG. 33B , the third mask MK 3  includes a plurality of via openings MK 3 -OP. The third mask MK 3  is arranged on the third dielectric film DF 3  so that the via openings MK 3 -OP reveals a portion of the third dielectric film DF 3 . In some embodiments, the third mask MK 3  includes a positive photo-resist or a negative photo-resist. 
     Referring to  FIG. 34 , in a subsequent step, the third dielectric film DF 3  is patterned using the third mask MK 3  to form third openings OP 3  in the third dielectric film DF 3 . For example, portions of the third dielectric film DF 3  not covered by the third mask MK 3  are removed through acceptable etching process. Thereafter, referring to  FIG. 35A  and  FIG. 35B , a third set of vias SLV 3  and a fourth set of vias SLV 4  (collectively referred as source line vias) are formed in the third openings OP 3  to fill up the third openings OP 3 . In some embodiments, the third set of vias SLV 3  and fourth set of vias SLV 4  are electrically connected to and in physical contact with the conductive pillars  108  (the second semiconductor region). Furthermore, the third dielectric film DF 3  (top dielectric film) is surrounding the third set of vias SLV 3  and the fourth set of vias SLV 4 , and covering and contacting a top surface of the conductive pillars  106  (the first semiconductor region) in each of the first sets of stacked memory cells  202 A and the second sets of stacked memory cells  202 B. 
     As further illustrated in  FIG. 35B , the third set of vias SLV 3  are arranged on the word line stack  58  and spaced apart from one another along the second direction D 2 . Furthermore, the third set of vias SLV 3  may be arranged in columns and spaced apart from one another along the first direction D  1 . In a similar way, the fourth set of vias SLV 4  are arranged on the word line stack  58  and spaced apart from one another along the second direction D 2 . Furthermore, the fourth set of vias SLV 4  may be arranged in columns and spaced apart from one another along the first direction D 1 . In the illustrated embodiment, the third set of vias SLV 3  are staggered widthwise in the second direction DL 2  from the fourth set of vias SLV 4 . In other words, the third set of vias SLV 3  and the fourth set of vias SLV 4  do not overlap one another in the second direction D 2 , or the third set of vias SLV 3  are misaligned with the fourth set of vias SLV 4 . In some embodiments, the third set of vias SLV 3  are connected to the conductive pillars  108  (the second semiconductor regions) on the first surface S 1  of the word line stack  58  (as defined in  FIG. 32A ), and non-overlapped with the first set of vias BLV 1  and the second set of vias BLV 2  in a build-up direction of the memory structure  200 . Similarly, the fourth set of vias SLV 4  are connected to the conductive pillars  108  (the second semiconductor regions) on the second surface of the word line stack  58 , and non-overlapped with the first set of vias BLV 1  and the second set of vias BLV 2  in the build-up direction of the memory structure  200 . 
     Referring to  FIG. 36A , after forming the third set of vias SLV 3  and the fourth set of vias SLV 4 , a fourth dielectric film DF 4  is formed over the third dielectric film DF 3  and over the third set of vias SLV 3  and the fourth set of vias SLV 4 . The fourth dielectric film DF 4  may be deposited by any suitable method, such as CVD, PEVCD, FCVD, or the like. In some embodiments, the fourth dielectric film DF 4  may be formed of materials such as phospho-silicate glass (PSG), boro-silicate glass (BSG), boron-doped phospho-silicate glass (BPSG), undoped silicate glass (USG), or the like. In some embodiments, the fourth dielectric film DF 4  may include an oxide (e.g., silicon oxide or the like), a nitride (e.g., silicon nitride or the like), a combination thereof or the like. Other dielectric materials formed by any acceptable process may be used. 
     After forming the fourth dielectric film DF 4 , a fourth mask MK 4  is formed thereon. For example, from a top view of the fourth mask MK 4  illustrated in  FIG. 36B , the fourth mask MK 4  includes a plurality of stripe patterns MK 4 -SP spaced apart from one another. The stripe patterns MK 4 -SP are arranged on the fourth dielectric film DF 4  so that a portion of the fourth dielectric film DF 4  is revealed or exposed. In some alternative embodiments, the fourth mask MK 4  include other patterns, which may be adjusted based on the routing designs of the source line array AX 2 . In some embodiments, the fourth mask MK 1  includes a positive photo-resist or a negative photo-resist. 
     Referring to  FIG. 37 , in a subsequent step, the fourth dielectric film DF 4  is patterned using the first mask MK 1  to form fourth openings OP 4  in the fourth dielectric film DF 4 . For example, portions of the fourth dielectric film DF 4  not covered by the fourth mask MK 4  are removed through acceptable etching process. Thereafter, referring to  FIG. 38A  and  FIG. 38B , a plurality of source line conductive layers SLC is formed in the fourth openings OP 4  to fill up the fourth openings OP 4 . For example, the source line conductive layers SLC are formed to be parallel to one another ( FIG. 38B ), but the disclosure is not limited thereto. In some embodiments, after the source line conductive layers SLC are deposited, a planarization process, such as a chemical mechanical polish (CMP), an etch-back process, combinations thereof, or the like may be performed. The planarization process is performed so that top surfaces of the source line conductive layers SLC are planar with a top surface of the fourth dielectric film DF 4 . Up to here, a source line array AX 2  is formed over the multi-layer stack  58  (or word line stack) and on the conductive pillars  106  and  108  (first and second semiconductor regions  106  and  108 ). 
     Referring to  FIG. 39A  to  FIG. 39C , in the staircase region, conductive contacts  110  are made on the conductive lines  72 , whereby the conductive contacts  110  are further connected to the conductive lines  116 C. In some embodiments, the staircase shape of the conductive lines  72  may provide a surface on each of the conductive lines  72  for the conductive contacts  110  to land on. For example, as shown in  FIG. 39B  and  FIG. 39C , the conductive contacts  110  may extend through the third dielectric film DF 3  and the IMD  70  to electrically connect conductive lines  116 C to the conductive lines  72  and the underlying active devices on the substrate. 
     In some embodiments, forming the contacts  110  may include patterning openings in the third dielectric film DF 3  and IMD  70  to expose portions of the conductive lines  72  using a combination of photolithography and etching, for example. A liner (not shown), such as a diffusion barrier layer, an adhesion layer, or the like, and a conductive material are formed in the openings. The liner may include titanium, titanium nitride, tantalum, tantalum nitride, or the like. The conductive material may include copper, a copper alloy, silver, gold, tungsten, cobalt, aluminum, nickel, or the like. A planarization process, such as a CMP, may be performed to remove excess material from the surface of the third dielectric film DF 3 . The remaining liner and conductive material form the contacts  110  in the openings. The conductive line  116 C may connect the memory structure to an underlying/overlying circuitry (e.g., control circuitry) and/or signal, power, and ground lines in the semiconductor die. Other conductive contacts or vias may be formed through the third dielectric film DF 3  and the fourth dielectric film of the source line array AX 2  to electrically connect the source line array AX 2  to the underlying active devices on the substrate. In alternate embodiments, routing and/or power lines to and from the memory structure may be provided by an interconnect structure formed over the memory structure  200  in addition to or in lieu of the interconnect structure  320  ( FIG. 2 ). Accordingly, the memory structure  200  may be completed. 
       FIG. 40  is a simplified sectional view of the memory structure  200  according to some exemplary embodiments of the present disclosure. As illustrated in  FIG. 40 , the memory structure  200  includes an intermediate region  1 RX having an active array BX 1  (or memory array) and a dummy array BX 2 . For example, the active array BX 1  includes the multi-layer stack  58  (or word line stack), the first sets of stacked memory cells  202 A and the second sets of stacked memory cells  202 B as illustrated in  FIG. 32A . The active array BX 1  is electrically connected to the bit line array AX 1  below and connected to the source line array AX 2  above. In some embodiments, the dummy array BX 2  includes the same structure made from the same fabrication process as with the structure of the active array BX 1 . However, the dummy array BX 2  is electrically floating. That is, the dummy array BX 2  is not connected to the bit line array AX 1  below and the source line array AX 2  above. 
     As further illustrated in  FIG. 40 , both of the bit line array AX 1  and the source line array AX 2  are electrically connected to the CMOS circuitry CX 1  shown in FIG. 2 . For example, the bit line array AX 1  is directly connected to the conductive features  322  of the interconnect structure  320 . Furthermore, the source line array AX 2  is electrically connected to the CMOS circuitry CX 1  through vias or connection structures that passes through a periphery (dielectrics or polymers) of the active array BX 1 , and through vias or connection structures that passes through the bit line array AX 1 . In other words, the vias or connections of the bit line array AX 1  and the source line array AX 2  will not extend to a region overlapping the dummy array BX 2 . 
     In the above-mentioned embodiments, the memory structure includes a bit line array disposed below the word line stack, and a source line array disposed above the word line stack. The conductive pillars (first and second semiconductor regions) located on two opposing surfaces of the word line stack are designed to be staggered from one another. As such, when the connections of the bit line array and the connections of the source line array are made to contact the conductive pillars (first and second semiconductor regions), the routing/connections of the source line array and the bit line array becomes relatively simplified. For example, the number of photomasks required to form the conductive layers of the bit line array and the source line array is reduced. Overall, a simplified process flow may be achieved, and the fabrication cost may be further reduced. 
     In accordance with some embodiments of the present disclosure, a memory structure includes a word line stack, first semiconductor regions and second semiconductor regions, a bit line array and a source line array. The word line stack has a first surface and a second surface opposite to the first surface. The first semiconductor regions and second semiconductor regions are alternately arranged along the first surface of the word line stack and spaced apart from one another in a first direction, and are alternately arranged along the second surface of the word line stack and spaced apart from one another in the first direction, wherein the first semiconductor regions and second semiconductor regions on the first surface are arranged to be staggered widthwise in a second direction from the first semiconductor regions and second semiconductor regions on the second surface, and the second direction being perpendicular to the first direction. The bit line array is disposed below the word line stack and connected to the first semiconductor regions on the first surface and the second surface of the word line stack. The source line array is disposed above the word line stack and connected to the second semiconductor regions on the first surface and the second surface of the word line stack. 
     In accordance with some other embodiments of the present disclosure, a memory structure includes first sets of stacked memory cells, second sets of stacked memory cells and a word line stack. The first sets of stacked memory cells are arranged along a first column in an active array of the memory structure. Each of the first sets of stacked memory cells includes a first conductive pillar and a second conductive pillar, a dielectric pillar and channel material layers. The dielectric pillar is disposed in between the first conductive pillar and the second conductive pillar. The channel material layers are located aside the first conductive pillar, the second conductive pillar and the dielectric pillar. The second sets of stacked memory cells are arranged along a second column in the active array of the memory structure. The second column is parallel to the first column. Each of the second sets of stacked memory cells includes, a third conductive pillar and a fourth conductive pillar, a second dielectric pillar and second channel materials. The third conductive pillar and the fourth conductive pillar are misaligned with the first conductive pillar and the second conductive pillar in the first sets of stacked memory cells. The second dielectric pillar disposed in between the third conductive pillar and the fourth conductive pillar. The second channel materials are located aside the third conductive pillar, the fourth conductive pillar and the second dielectric pillar. The word line stack is arranged along a third column in the active array between the first column and the second column, and disposed in between the first sets of stacked memory cells and the second sets of stacked memory cell. 
     In accordance with some embodiments of the present disclosure, a method of fabricating a memory structure is described. The method includes the following steps. A CMOS circuitry is formed over a semiconductor substrate. A bit line array is formed to be electrically connected to the CMOS circuitry. A memory array is formed over the bit line array, wherein the memory array is formed by forming a word line stack, and forming first and second sets of stacked memory cells. The word line stack is formed on the bit line array, wherein the word line stack has a first side surface and a second side surface opposite to the first side surface. The first sets of sacked memory cells are formed along the first side surface. The second sets of stacked memory cells are formed along the second side surface, wherein the second sets of stacked memory cells are staggered from the first sets of stacked memory cells. A source line array is formed over the memory array, wherein the source line array is electrically connected to the CMOS circuitry through connection structures passing through the bit line array. 
     In accordance with some other embodiments of the present disclosure, a method of fabricating a memory structure is described. The method includes the following steps. A bit line array is formed over a substrate. The bit line array is formed by the following steps. A first dielectric film is formed over the substrate, wherein the first dielectric film includes first openings. Bit line conductive layers are formed in the first openings. A second dielectric film is formed over the first dielectric film and over the bit line conductive layers, wherein the second dielectric film includes second openings. A first set of vias and a second set of vias are formed in the second openings, wherein the first set of vias and the second set of vias are staggered from one another. A plurality of memory cells is formed on the bit line array, wherein the plurality of memory cells comprises first electrode layers and second electrode layers, and the first electrode layers are electrically connected to the bit line array. A source line array is formed over the plurality of memory cells, wherein the source line array is electrically connected to the second electrode layers. 
     In accordance with yet another embodiment of the present disclosure, a method including the following steps is described. A substrate is provided. A first dielectric film is formed over the substrate, and the first dielectric film is patterned using a first mask. Bit line conductive layers are formed in first openings defined by the first mask. A second dielectric film is formed over the first dielectric film, and the second dielectric film is patterned using a second mask. Bit line vias are formed in second openings defined by the second mask, wherein the bit line vias have a staggered arrangement. A memory array is formed on the second dielectric film and electrically connected to the bit line vias. A third dielectric film is formed on the memory array, and the third dielectric film is patterned using a third mask. Source line vias are formed in third openings defined by the third mask, wherein the source line vias have a staggered arrangement and are electrically connected to the memory array. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.