# Patent

## Title
Vertical thin film transistors with surround gates

## Abstract

A method is provided that includes forming a transistor by forming a gate disposed in a first direction above a substrate, the gate including a first bridge portion and a second bridge portion, forming the first bridge portion extending in the first direction and disposed near a top of the gate, and forming the second bridge portion extending in the first direction and disposed near a bottom of the gate.

## Background
BACKGROUND
Semiconductor memory is widely used in various electronic devices such as mobile computing devices, mobile phones, solid-state drives, digital cameras, personal digital assistants, medical electronics, servers, and non-mobile computing devices. Semiconductor memory may include non-volatile memory or volatile memory. A non-volatile memory device allows information to be stored or retained even when the non-volatile memory device is not connected to a source of power (e.g., a battery).
Examples of non-volatile memory include flash memory (e.g., NAND-type and NOR-type flash memory), Electrically Erasable Programmable Read-Only Memory (EEPROM), ferroelectric memory (e.g., FeRAM), magnetoresistive memory (e.g., MRAM), and phase change memory (e.g., PRAM). In recent years, non-volatile memory devices have been scaled in order to reduce the cost per bit. However, as process geometries shrink, many design and process challenges are presented. These challenges include increased variability in memory cell I-V characteristics, reduced memory cell sensing currents, and increased bit line settling times.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A depicts an embodiment of a memory system and a host.
FIG. 1B depicts an embodiment of memory core control circuits.
FIG. 1C depicts an embodiment of a memory core.
FIG. 1D depicts an embodiment of a memory bay.
FIG. 1E depicts an embodiment of a memory block.
FIG. 1F depicts another embodiment of a memory bay.
FIG. 2A depicts a schematic diagram of the memory bay of FIG. 1F.
FIG. 2B depicts a schematic diagram of a memory bay arrangement wherein word lines and bit lines are shared across memory blocks, and both row decoders and column decoders are split.
FIG. 3A depicts an embodiment of a portion of a monolithic three-dimensional memory array.
FIG. 3B depicts a subset of the memory array and routing layers of an embodiment of a three-dimensional memory array.
FIG. 4A depicts an embodiment of a portion of a monolithic three-dimensional memory array.
FIG. 4B depicts an embodiment of a portion of a monolithic three-dimensional memory array that includes vertical strips of a non-volatile memory material.
FIGS. 5A-5D depict various views of an embodiment of a monolithic three-dimensional memory array.
FIGS. 6A1-6L2 are cross-sectional views of a portion of a substrate during an example fabrication of the monolithic three-dimensional memory array of FIGS. 5A-5D.
FIGS. 7A-7E depict various views of another embodiment of a monolithic three-dimensional memory array.
FIGS. 8A1-8J5 are cross-sectional views of a portion of a substrate during an example fabrication of the monolithic three-dimensional memory array of FIGS. 7A-7E.



DETAILED DESCRIPTION
Technology is described for monolithic three-dimensional memory arrays. In particular, a monolithic three-dimensional memory array is formed that includes a vertically-oriented bit line select transistor. A method is provided for forming the vertically-oriented bit line select transistor by forming a gate disposed in a first direction above a substrate. The gate includes a first bridge portion and a second bridge portion. The first bridge portion extends in the first direction and is disposed near a top of the gate. The second bridge portion extends in the first direction and is disposed near a bottom of the gate.
In some embodiments, a memory array may include a cross-point memory array. A cross-point memory array may refer to a memory array in which two-terminal memory cells are placed at the intersections of a first set of control lines (e.g., word lines) arranged in a first direction and a second set of control lines (e.g., bit lines) arranged in a second direction perpendicular to the first direction. The two-terminal memory cells may include a resistance-switching material, such as a phase change material, a ferroelectric material, or a metal oxide (e.g., nickel oxide or hafnium oxide). In some cases, each memory cell in a cross-point memory array may be placed in series with a steering element or an isolation element, such as a diode, to reduce leakage currents. In cross-point memory arrays where the memory cells do not include an isolation element, controlling and minimizing leakage currents may be a significant issue, especially because leakage currents may vary greatly over biasing voltage and temperature.
In one embodiment, a non-volatile storage system may include one or more two-dimensional arrays of non-volatile memory cells. The memory cells within a two-dimensional memory array may form a single layer of memory cells and may be selected via control lines (e.g., word lines and bit lines) in the X and Y directions. In another embodiment, a non-volatile storage system may include one or more monolithic three-dimensional memory arrays in which two or more layers of memory cells may be formed above a single substrate without any intervening substrates. In some cases, a three-dimensional memory array may include one or more vertical columns of memory cells located above and orthogonal to a substrate. In one example, a non-volatile storage system may include a memory array with vertical bit lines or bit lines that are arranged orthogonal to a semiconductor substrate. The substrate may include a silicon substrate. The memory array may include rewriteable non-volatile memory cells, wherein each memory cell includes a reversible resistance-switching element without an isolation element in series with the reversible resistance-switching element (e.g., no diode in series with the reversible resistance-switching element).
In some embodiments, a non-volatile storage system may include a non-volatile memory that is monolithically formed in one or more physical levels of arrays of memory cells having an active area disposed above a silicon substrate. The non-volatile storage system may also include circuitry associated with the operation of the memory cells (e.g., decoders, state machines, page registers, or control circuitry for controlling the reading or programming of the memory cells). The circuitry associated with the operation of the memory cells may be located above the substrate or located within the substrate.
In some embodiments, a non-volatile storage system may include a monolithic three-dimensional memory array. The monolithic three-dimensional memory array may include one or more levels of memory cells. Each memory cell within a first level of the one or more levels of memory cells may include an active area that is located above a substrate (e.g., above a single-crystal substrate or a crystalline silicon substrate). In one example, the active area may include a semiconductor junction (e.g., a P-N junction). The active area may include a portion of a source or drain region of a transistor. In another example, the active area may include a channel region of a transistor.
FIG. 1A depicts one embodiment of a memory system 100 and a host 102. Memory system 100 may include a non-volatile storage system interfacing with host 102 (e.g., a mobile computing device). In some cases, memory system 100 may be embedded within host 102. In other cases, memory system 100 may include a memory card. As depicted, memory system 100 includes a memory chip controller 104 and a memory chip 106. Although a single memory chip 106 is depicted, memory system 100 may include more than one memory chip (e.g., four, eight or some other number of memory chips). Memory chip controller 104 may receive data and commands from host 102 and provide memory chip data to host 102.
Memory chip controller 104 may include one or more state machines, page registers, SRAM, and control circuitry for controlling the operation of memory chip 106. The one or more state machines, page registers, SRAM, and control circuitry for controlling the operation of memory chip 106 may be referred to as managing or control circuits. The managing or control circuits may facilitate one or more memory array operations, such as forming, erasing, programming, or reading operations.
In some embodiments, the managing or control circuits (or a portion of the managing or control circuits) for facilitating one or more memory array operations may be integrated within memory chip 106. Memory chip controller 104 and memory chip 106 may be arranged on a single integrated circuit. In other embodiments, memory chip controller 104 and memory chip 106 may be arranged on different integrated circuits. In some cases, memory chip controller 104 and memory chip 106 may be integrated on a system board, logic board, or a PCB.
Memory chip 106 includes memory core control circuits 108 and a memory core 110. Memory core control circuits 108 may include logic for controlling the selection of memory blocks (or arrays) within memory core 110, controlling the generation of voltage references for biasing a particular memory array into a read or write state, or generating row and column addresses.
Memory core 110 may include one or more two-dimensional arrays of memory cells or one or more three-dimensional arrays of memory cells. In one embodiment, memory core control circuits 108 and memory core 110 are arranged on a single integrated circuit. In other embodiments, memory core control circuits 108 (or a portion of memory core control circuits 108) and memory core 110 may be arranged on different integrated circuits.
A memory operation may be initiated when host 102 sends instructions to memory chip controller 104 indicating that host 102 would like to read data from memory system 100 or write data to memory system 100. In the event of a write (or programming) operation, host 102 will send to memory chip controller 104 both a write command and the data to be written. The data to be written may be buffered by memory chip controller 104 and error correcting code (ECC) data may be generated corresponding with the data to be written. The ECC data, which allows data errors that occur during transmission or storage to be detected and/or corrected, may be written to memory core 110 or stored in non-volatile memory within memory chip controller 104. In one embodiment, the ECC data are generated and data errors are corrected by circuitry within memory chip controller 104.
Memory chip controller 104 controls operation of memory chip 106. In one example, before issuing a write operation to memory chip 106, memory chip controller 104 may check a status register to make sure that memory chip 106 is able to accept the data to be written. In another example, before issuing a read operation to memory chip 106, memory chip controller 104 may pre-read overhead information associated with the data to be read. The overhead information may include ECC data associated with the data to be read or a redirection pointer to a new memory location within memory chip 106 in which to read the data requested. Once a read or write operation is initiated by memory chip controller 104, memory core control circuits 108 may generate the appropriate bias voltages for word lines and bit lines within memory core 110, and generate the appropriate memory block, row, and column addresses.
In some embodiments, one or more managing or control circuits may be used for controlling the operation of a memory array. The one or more managing or control circuits may provide control signals to a memory array to perform a read operation and/or a write operation on the memory array. In one example, the one or more managing or control circuits may include any one of or a combination of control circuitry, state machine, decoders, sense amplifiers, read/write circuits, and/or controllers. The one or more managing circuits may perform or facilitate one or more memory array operations including erasing, programming, or reading operations. In one example, one or more managing circuits may include an on-chip memory controller for determining row and column address, word line and bit line addresses, memory array enable signals, and data latching signals.
FIG. 1B depicts an embodiment of memory core control circuits 108. As depicted, memory core control circuits 108 include address decoders 120, voltage generators for selected control lines 122, voltage generators for unselected control lines 124 and signal generators for reference signals 126 (described in more detail below). Control lines may include word lines, bit lines, or a combination of word lines and bit lines. Selected control lines may include selected word lines and/or selected bit lines that are used to place memory cells into a selected state. Unselected control lines may include unselected word lines and/or unselected bit lines that are used to place memory cells into an unselected state.
Address decoders 120 may generate memory block addresses, as well as row addresses and column addresses for a particular memory block. Voltage generators (or voltage regulators) for selected control lines 122 may include one or more voltage generators for generating selected control line voltages. Voltage generators for unselected control lines 124 may include one or more voltage generators for generating unselected control line voltages. Signal generators for reference signals 126 may include one or more voltage and/or current generators for generating reference voltage and/or current signals.
FIGS. 1C-1F depict one embodiment of a memory core organization that includes a memory core having multiple memory bays, and each memory bay having multiple memory blocks. Although a memory core organization is disclosed where memory bays include memory blocks, and memory blocks include a group of memory cells, other organizations or groupings also can be used with the technology described herein.
FIG. 1C depicts one embodiment of memory core 110 in FIG. 1A. As depicted, memory core 110 includes memory bay 130 and memory bay 132. In some embodiments, the number of memory bays per memory core can be different for different implementations. For example, a memory core may include only a single memory bay or multiple memory bays (e.g., 16 or other number of memory bays).
FIG. 1D depicts one embodiment of memory bay 130 in FIG. 1C. As depicted, memory bay 130 includes memory blocks 140-144 and read/write circuits 146. In some embodiments, the number of memory blocks per memory bay may be different for different implementations. For example, a memory bay may include one or more memory blocks (e.g., 32 or other number of memory blocks per memory bay). Read/write circuits 146 include circuitry for reading and writing memory cells within memory blocks 140-144.
As depicted, read/write circuits 146 may be shared across multiple memory blocks within a memory bay. This allows chip area to be reduced because a single group of read/write circuits 146 may be used to support multiple memory blocks. However, in some embodiments, only a single memory block may be electrically coupled to read/write circuits 146 at a particular time to avoid signal conflicts.
In some embodiments, read/write circuits 146 may be used to write one or more pages of data into memory blocks 140-144 (or into a subset of the memory blocks). The memory cells within memory blocks 140-144 may permit direct over-writing of pages (i.e., data representing a page or a portion of a page may be written into memory blocks 140-144 without requiring an erase or reset operation to be performed on the memory cells prior to writing the data).
In one example, memory system 100 in FIG. 1A may receive a write command including a target address and a set of data to be written to the target address. Memory system 100 may perform a read-before-write (RBW) operation to read the data currently stored at the target address and/or to acquire overhead information (e.g., ECC information) before performing a write operation to write the set of data to the target address.
In some cases, read/write circuits 146 may be used to program a particular memory cell to be in one of three or more data/resistance states (i.e., the particular memory cell may include a multi-level memory cell). In one example, read/write circuits 146 may apply a first voltage difference (e.g., 2V) across the particular memory cell to program the particular memory cell into a first state of the three or more data/resistance states or a second voltage difference (e.g., 1V) across the particular memory cell that is less than the first voltage difference to program the particular memory cell into a second state of the three or more data/resistance states.
Applying a smaller voltage difference across the particular memory cell may cause the particular memory cell to be partially programmed or programmed at a slower rate than when applying a larger voltage difference. In another example, read/write circuits 146 may apply a first voltage difference across the particular memory cell for a first time period (e.g., 150 ns) to program the particular memory cell into a first state of the three or more data/resistance states or apply the first voltage difference across the particular memory cell for a second time period less than the first time period (e.g., 50 ns). One or more programming pulses followed by a memory cell verification phase may be used to program the particular memory cell to be in the correct state.
FIG. 1E depicts one embodiment of memory block 140 in FIG. 1D. As depicted, memory block 140 includes a memory array 150, row decoder 152, and column decoder 154. Memory array 150 may include a contiguous group of memory cells having contiguous word lines and bit lines. Memory array 150 may include one or more layers of memory cells. Memory array 150 may include a two-dimensional memory array or a three-dimensional memory array.
Row decoder 152 decodes a row address and selects a particular word line in memory array 150 when appropriate (e.g., when reading or writing memory cells in memory array 150). Column decoder 154 decodes a column address and selects one or more bit lines in memory array 150 to be electrically coupled to read/write circuits, such as read/write circuits 146 in FIG. 1D. In one embodiment, the number of word lines is 4K per memory layer, the number of bit lines is 1K per memory layer, and the number of memory layers is 4, providing a memory array 150 containing 16 L memory cells.
FIG. 1F depicts one embodiment of a memory bay 134. Memory bay 134 is one example of an alternative implementation for memory bay 130 in FIG. 1D. In some embodiments, row decoders, column decoders, and read/write circuits may be split or shared between memory arrays. As depicted, row decoder 152b is shared between memory arrays 150a and 150b because row decoder 152b controls word lines in both memory arrays 150a and 150b (i.e., the word lines driven by row decoder 152b are shared).
Row decoders 152a and 152b may be split such that even word lines in memory array 150a are driven by row decoder 152a and odd word lines in memory array 150a are driven by row decoder 152b. Row decoders 152c and 152b may be split such that even word lines in memory array 150b are driven by row decoder 152c and odd word lines in memory array 150b are driven by row decoder 152b. 
Column decoders 154a and 154b may be split such that even bit lines in memory array 150a are controlled by column decoder 154b and odd bit lines in memory array 150a are driven by column decoder 154a. Column decoders 154c and 154d may be split such that even bit lines in memory array 150b are controlled by column decoder 154d and odd bit lines in memory array 150b are driven by column decoder 154c. 
The selected bit lines controlled by column decoder 154a and column decoder 154c may be electrically coupled to read/write circuits 146a. The selected bit lines controlled by column decoder 154b and column decoder 154d may be electrically coupled to read/write circuits 146b. Splitting the read/write circuits into read/write circuits 146a and 146b when the column decoders are split may allow for a more efficient layout of the memory bay.
FIG. 2A depicts one embodiment of a schematic diagram (including word lines and bit lines) corresponding with memory bay 134 in FIG. 1F. As depicted, word lines WL1, WL3, and WL5 are shared between memory arrays 150a and 150b and controlled by row decoder 152b of FIG. 1F. Word lines WL0, WL2, WL4, and WL6 are driven from the left side of memory array 150a and controlled by row decoder 152a of FIG. 1F. Word lines WL14, WL16, WL18, and WL20 are driven from the right side of memory array 150b and controlled by row decoder 152c of FIG. 1F.
Bit lines BL0, BL2, BL4, and BL6 are driven from the bottom of memory array 150a and controlled by column decoder 154b of FIG. 1F. Bit lines BL1, BL3, and BL5 are driven from the top of memory array 150a and controlled by column decoder 154a of FIG. 1F. Bit lines BL7, BL9, BL11, and BL13 are driven from the bottom of memory array 150b and controlled by column decoder 154d of FIG. 1F. Bit lines BL8, BL10, and BL12 are driven from the top of memory array 150b and controlled by column decoder 154c of FIG. 1F.
In one embodiment, memory arrays 150a and 150b may include memory layers that are oriented in a horizontal plane that is horizontal to the supporting substrate. In another embodiment, memory arrays 150a and 150b may include memory layers that are oriented in a vertical plane that is vertical with respect to the supporting substrate (i.e., the vertical plane is perpendicular to the supporting substrate).
FIG. 2B depicts one embodiment of a schematic diagram (including word lines and bit lines) corresponding with a memory bay arrangement wherein word lines and bit lines are shared across memory blocks, and both row decoders and column decoders are split. Sharing word lines and/or bit lines helps to reduce layout area since a single row decoder and/or column decoder can be used to support two memory arrays.
As depicted, word lines WL1, WL3, and WL5 are shared between memory arrays 150a and 150b, and word lines WL8, WL10 and WL12 are shared between memory arrays 150c and 150d. Bit lines BL1, BL3, and BL5 are shared between memory arrays 150a and 150c, and bit lines BL8, BL10, and BL12 are shared between memory arrays 150b and 150d. 
Row decoders are split such that word lines WL0, WL2, WL4, and WL6 are driven from the left side of memory array 150a and word lines WL1, WL3, and WL5 are driven from the right side of memory array 150a. Likewise, word lines WL7, WL9, WL11, and WL13 are driven from the left side of memory array 150c and word lines WL8, WL10, and WL12 are driven from the right side of memory array 150c. 
Column decoders are split such that bit lines BL0, BL2, BL4, and BL6 are driven from the bottom of memory array 150a and bit lines BL1, BL3, and BL5 are driven from the top of memory array 150a. Likewise, bit lines BL21, BL23, BL25, and BL27 are driven from the top of memory array 150d and bit lines BL8, BL10, and BL12 are driven from the bottom of memory array 150d. Splitting row and/or column decoders also helps to relieve layout constraints (e.g., the column decoder pitch can be relieved by 2x since the split column decoders need only drive every other bit line instead of every bit line).
FIG. 3A depicts one embodiment of a portion of a monolithic three-dimensional memory array 300 that includes a first memory level 302 and a second memory level 304 positioned above first memory level 302. Memory array 300 is one example of an implementation for memory array 150 in FIG. 1E. Bit lines 306 and 308 are arranged in a first direction and word lines 310 are arranged in a second direction perpendicular to the first direction. As depicted, the upper conductors of first memory level 302 may be used as the lower conductors of second memory level 304. In a memory array with additional layers of memory cells, there would be corresponding additional layers of bit lines and word lines.
Memory array 300 includes a plurality of memory cells 312. Memory cells 312 may include re-writeable memory cells, and may include non-volatile memory cells or volatile memory cells. With respect to first memory level 302, a first portion of memory cells 312 are between and connect to bit lines 306 and word lines 310. With respect to second memory level 304, a second portion of memory cells 312 are between and connect to bit lines 308 and word lines 310. In one embodiment, each memory cell 312 includes a steering element (e.g., a diode) and a memory element (i.e., a state change element).
In one example, the diodes of first memory level 302 may be upward pointing diodes as indicated by arrow A1 (e.g., with p regions at the bottom of the diodes), whereas the diodes of second memory level 304 may be downward pointing diodes as indicated by arrow A2 (e.g., with n regions at the bottom of the diodes), or vice versa. In another embodiment, each memory cell 312 includes only a state change element. The absence of a diode (or other steering element) from a memory cell may reduce the process complexity and costs associated with manufacturing a memory array.
In one embodiment, memory cells 312 include re-writable non-volatile memory cells including a reversible resistance-switching element. A reversible resistance-switching element may include a reversible resistance-switching material having a resistance that may be reversibly switched between two or more states. In one embodiment, the reversible resistance-switching material may include a metal oxide (e.g., a binary metal oxide). The metal oxide may include nickel oxide, hafnium oxide, or some other metal oxide material. In another embodiment, the reversible resistance-switching material may include a phase change material. The phase change material may include a chalcogenide material. In some cases, the re-writeable non-volatile memory cells may include resistive RAM (ReRAM) devices.
In another embodiment, memory cells 312 may include conductive bridge memory elements. A conductive bridge memory element also may be referred to as a programmable metallization cell. A conductive bridge memory element may be used as a state change element based on the physical relocation of ions within a solid electrolyte. In some cases, a conductive bridge memory element may include two solid metal electrodes, one relatively inert (e.g., tungsten) and the other electrochemically active (e.g., silver or copper), with a thin film of the solid electrolyte between the two electrodes. As temperature increases, the mobility of the ions also increases causing the programming threshold for the conductive bridge memory cell to decrease. Thus, the conductive bridge memory element may have a wide range of programming thresholds over temperature.
In one embodiment of a read operation, the data stored in one of the plurality of memory cells 312 may be read by biasing one of the word lines (i.e., the selected word line) to a selected word line voltage in read mode (e.g., 0V). A sense amplifier may then be used to bias a selected bit line connected to the selected memory cell to the selected bit line voltage in read mode (e.g., 1.0V). In some cases, to avoid sensing leakage current from the many unselected word lines to the selected bit line, the unselected word lines may be biased to the same voltage as the selected bit lines (e.g., 1.0V). To avoid leakage current from the selected word line to the unselected bit lines, the unselected bit lines may be biased to the same voltage as the selected word line (e.g., 0V). However, biasing the unselected word lines to the same voltage as the selected bit lines and biasing the unselected bit lines to the same voltage as the selected word line may place a substantial voltage stress across the unselected memory cells driven by both the unselected word lines and the unselected bit lines.
In an alternative read biasing scheme, both the unselected word lines and the unselected bit lines may be biased to an intermediate voltage that is between the selected word line voltage and the selected bit line voltage. Applying the same voltage to both the unselected word lines and the unselected bit lines may reduce the voltage stress across the unselected memory cells driven by both the unselected word lines and the unselected bit lines.
However, the reduced voltage stress comes at the expense of increased leakage currents associated with the selected word line and the selected bit line. Before the selected word line voltage has been applied to the selected word line, the selected bit line voltage may be applied to the selected bit line, and a sense amplifier may then sense an auto zero amount of current through the selected memory bit line which is subtracted from the bit line current in a second current sensing when the selected word line voltage is applied to the selected word line. The leakage current may be subtracted out by using the auto zero current sensing.
In one embodiment of a write operation, the reversible resistance-switching material may be in an initial high-resistance state that is switchable to a low-resistance state upon application of a first voltage and/or current. Application of a second voltage and/or current may return the reversible resistance-switching material back to the high-resistance state. Alternatively, the reversible resistance-switching material may be in an initial low-resistance state that is reversibly switchable to a high-resistance state upon application of the appropriate voltage(s) and/or current(s).
When used in a memory cell, one resistance state may represent a binary data “0,” and another resistance state may represent a binary data “1.” In some cases, a memory cell may be considered to include more than two data/resistance states (i.e., a multi-level memory cell). In some cases, a write operation may be similar to a read operation except with a larger voltage range placed across the selected memory cells.
The process of switching the resistance of a reversible resistance-switching element from a high-resistance state to a low-resistance state may be referred to as SETTING the reversible resistance-switching element. The process of switching the resistance from the low-resistance state to the high-resistance state may be referred to as RESETTING the reversible resistance-switching element. The high-resistance state may be associated with binary data “1” and the low-resistance state may be associated with binary data “0.” In other embodiments, SETTING and RESETTING operations and/or the data encoding can be reversed. In some embodiments, the first time a resistance-switching element is SET may require a higher than normal programming voltage and may be referred to as a FORMING operation.
In one embodiment of a write operation, data may be written to one of the plurality of memory cells 312 by biasing one of the word lines (i.e., the selected word line) to the selected word line voltage in write mode (e.g., 5V). A write circuit may be used to bias the bit line connected to the selected memory cell to the selected bit line voltage in write mode (e.g., 0V).
In some cases, to prevent program disturb of unselected memory cells sharing the selected word line, the unselected bit lines may be biased such that a first voltage difference between the selected word line voltage and the unselected bit line voltage is less than a first disturb threshold. To prevent program disturb of unselected memory cells sharing the selected bit line, the unselected word lines may be biased such that a second voltage difference between the unselected word line voltage and the selected bit line voltage is less than a second disturb threshold. The first disturb threshold and the second disturb threshold may be different depending on the amount of time in which the unselected memory cells susceptible to disturb are stressed.
In one write biasing scheme, both the unselected word lines and the unselected bit lines may be biased to an intermediate voltage that is between the selected word line voltage and the selected bit line voltage. The intermediate voltage may be generated such that a first voltage difference across unselected memory cells sharing a selected word line is greater than a second voltage difference across other unselected memory cells sharing a selected bit line. One reason for placing the larger voltage difference across the unselected memory cells sharing a selected word line is that the memory cells sharing the selected word line may be verified immediately after a write operation to detect a write disturb.
FIG. 3B depicts a subset of the memory array and routing layers of one embodiment of a three-dimensional memory array, such as memory array 150 in FIG. 1E. As depicted, the Memory Array layers are positioned above the Substrate. The Memory Array layers include bit line layers BL0, BL1 and BL2, and word line layers WL0 and WL1. In other embodiments, additional bit line and word line layers also can be implemented. Supporting circuitry (e.g., row decoders, column decoders, and read/write circuits) may be arranged on the surface of the Substrate with the Memory Array layers fabricated above the supporting circuitry.
An integrated circuit implementing a three-dimensional memory array also may include multiple metal layers for routing signals between different components of the supporting circuitry, and between the supporting circuitry and the bit lines and word lines of the memory array. These routing layers can be arranged above the supporting circuitry that is implemented on the surface of the Substrate and below the Memory Array layers.
As depicted in FIG. 3B, two metal layers R1 and R2 are used for routing layers. However, other embodiments can include more or less than two metal layers. In one example, these metal layers R1 and R2 are formed of tungsten (about 1 ohm/square). Positioned above the Memory Array layers may be one or more top metal layers used for routing signals between different components of the integrated circuit, such as the Top Metal layer. In one example, the Top Metal layer is formed of copper or aluminum (about 0.05 ohms/square), which may provide a smaller resistance per unit area than metal layers R1 and R2. In some cases, metal layers R1 and R2 may not be implemented using the same materials as those used for the Top Metal layers because the metal used for R1 and R2 must be able to withstand the processing steps for fabricating the Memory Array layers on top of R1 and R2 (e.g., satisfying a particular thermal budget during fabrication).
FIG. 4A depicts one embodiment of a portion of a monolithic three-dimensional memory array 400 that includes a first memory level 410, and a second memory level 412 positioned above first memory level 410. Memory array 400 is one example of an implementation for memory array 150 in FIG. 1E. Local bit lines LBL11-LBL33 are arranged in a first direction (e.g., a z-direction) and word lines WL10-WL23 are arranged in a second direction (e.g., an x-direction) perpendicular to the first direction. This arrangement of vertical bit lines in a monolithic three-dimensional memory array is one embodiment of a vertical bit line memory array.
As depicted, disposed between the intersection of each local bit line and each word line is a particular memory cell (e.g., memory cell M111 is disposed between local bit line LBL11 and word line WL10). The particular memory cell may include a floating gate device, a charge trap device (e.g., using a silicon nitride material), a reversible resistance-switching device, a ReRAM device, or other similar device. The global bit lines GBL1-GBL3 are arranged in a third direction (e.g., a y-direction) that is perpendicular to both the first direction and the second direction.
Each local bit line LBL11-LBL33 has an associated bit line select transistor Q11-Q33, respectively. Bit line select transistors Q11-Q33 may be field effect transistors, such as shown, or may be any other transistors. As depicted, bit line select transistors Q11-Q31 are associated with local bit lines LBL11-LBL31, respectively, and may be used to connect local bit lines LBL11-LBL31 to global bit lines GBL1-GBL3, respectively, using row select line SG1. In particular, each of bit line select transistors Q11-Q31 has a first terminal (e.g., a drain/source terminal) coupled to a corresponding one of local bit lines LBL11-LBL31, respectively, a second terminal (e.g., a source/drain terminal) coupled to a corresponding one of global bit lines GBL1-GBL3, respectively, and a third terminal (e.g., a gate terminal) coupled to row select line SG1.
Similarly, bit line select transistors Q12-Q32 are associated with local bit lines LBL12-LBL32, respectively, and may be used to connect local bit lines LBL12-LBL32 to global bit lines GBL1-GBL3, respectively, using row select line SG2. In particular, each of bit line select transistors Q12-Q32 has a first terminal (e.g., a drain/source terminal) coupled to a corresponding one of local bit lines LBL12-LBL32, respectively, a second terminal (e.g., a source/drain terminal) coupled to a corresponding one of global bit lines GBL1-GBL3, respectively, and a third terminal (e.g., a gate terminal) coupled to row select line SG2.
Likewise, bit line select transistors Q13-Q33 are associated with local bit lines LBL13-LBL33, respectively, and may be used to connect local bit lines LBL13-LBL33 to global bit lines GBL1-GBL3, respectively, using row select line SG3. In particular, each of bit line select transistors Q13-Q33 has a first terminal (e.g., a drain/source terminal) coupled to a corresponding one of local bit lines LBL13-LBL33, respectively, a second terminal (e.g., a source/drain terminal) coupled to a corresponding one of global bit lines GBL1-GBL3, respectively, and a third terminal (e.g., a gate terminal) coupled to row select line SG3.
Because a single bit line select transistor is associated with a corresponding local bit line, the voltage of a particular global bit line may be applied to a corresponding local bit line. Therefore, when a first set of local bit lines (e.g., LBL11-LBL31) is biased to global bit lines GBL1-GBL3, the other local bit lines (e.g., LBL12-LBL32 and LBL13-LBL33) must either also be driven to the same global bit lines GBL1-GBL3 or be floated.
In one embodiment, during a memory operation, all local bit lines within the memory array are first biased to an unselected bit line voltage by connecting each of the global bit lines to one or more local bit lines. After the local bit lines are biased to the unselected bit line voltage, then only a first set of local bit lines LBL11-LBL31 are biased to one or more selected bit line voltages via the global bit lines GBL1-GBL3, while the other local bit lines (e.g., LBL12-LBL32 and LBL13-LBL33) are floated. The one or more selected bit line voltages may correspond with, for example, one or more read voltages during a read operation or one or more programming voltages during a programming operation.
In one embodiment, a vertical bit line memory array, such as memory array 400, includes a greater number of memory cells along the word lines as compared with the number of memory cells along the vertical bit lines (e.g., the number of memory cells along a word line may be more than 10 times the number of memory cells along a bit line). In one example, the number of memory cells along each bit line may be 16 or 32, whereas the number of memory cells along each word line may be 2048 or more than 4096. Other numbers of memory cells along each bit line and along each word line may be used.
In one embodiment of a read operation, the data stored in a selected memory cell (e.g., memory cell M111) may be read by biasing the word line connected to the selected memory cell (e.g., selected word line WL10) to a selected word line voltage in read mode (e.g., 0V). The local bit line (e.g., LBL11) coupled to the selected memory cell (M111) is biased to a selected bit line voltage in read mode (e.g., 1 V) via the associated bit line select transistor (e.g., Q11) coupled to the selected local bit line (LBL11), and the global bit line (e.g., GBL1) coupled to the bit line select transistor (Q11). A sense amplifier may then be coupled to the selected local bit line (LBL11) to determine a read current IREAD of the selected memory cell (M111). The read current IREAD is conducted by the bit line select transistor Q11, and may be between about 100 nA and about 500 nA, although other read currents may be used.
In one embodiment of a write operation, data may be written to a selected memory cell (e.g., memory cell M221) by biasing the word line connected to the selected memory cell (e.g., WL20) to a selected word line voltage in write mode (e.g., 5V). The local bit line (e.g., LBL21) coupled to the selected memory cell (M221) is biased to a selected bit line voltage in write mode (e.g., 0 V) via the associated bit line select transistor (e.g., Q21) coupled to the selected local bit line (LBL21), and the global bit line (e.g., GBL2) coupled to the bit line select transistor (Q21). During a write operation, a programming current IPGRm is conducted by the associated bit line select transistor Q21, and may be between about 3 uA and about 6 uA, although other programming currents may be used.
FIG. 4B depicts an embodiment of a portion of a monolithic three-dimensional memory array that includes vertical strips of a non-volatile memory material. The physical structure depicted in FIG. 4B may include one implementation for a portion of the monolithic three-dimensional memory array depicted in FIG. 4A. The vertical strips of non-volatile memory material may be formed in a direction that is perpendicular to a substrate (e.g., in the z-direction). A vertical strip of the non-volatile memory material 414 may include, for example, a vertical oxide layer, a vertical reversible resistance-switching material (e.g., a metal oxide material, a phase change material or other similar reversible resistance-switching material), or a vertical charge trapping layer (e.g., a layer of silicon nitride). The vertical strip of material may include a single continuous layer of material that may be used by a plurality of memory cells or devices. In an embodiment, vertical strip of non-volatile memory material 414 includes one or more of HfO2, Al2O3, HfSiOx, HfSiOxNy, HfAlOx, Nb2O5, Ta2O5, ZrO2, Cr2O3, Fe2O3, Ni2O3, CO2O3, WO3, TiO2, SrZrO3, SrTiO3.
In one example, portions of the vertical strip of the non-volatile memory material 414 may include a part of a first memory cell associated with the cross section between WL12 and LBL13 and a part of a second memory cell associated with the cross section between WL22 and LBL13. In some cases, a vertical bit line, such as LBL13, may include a vertical structure (e.g., a rectangular prism, a cylinder, or a pillar) and the non-volatile material may completely or partially surround the vertical structure (e.g., a conformal layer of phase change material surrounding the sides of the vertical structure).
As depicted, each of the vertical bit lines may be connected to one of a set of global bit lines via an associated vertically-oriented bit line select transistor (e.g., Q11, Q12, Q13, Q23). Each vertically-oriented bit line select transistor may include a MOS device (e.g., an NMOS device) or a vertical thin-film transistor (TFT).
In an embodiment, each vertically-oriented bit line select transistor is a vertically-oriented pillar-shaped TFT coupled between an associated local bit line pillar and a global bit line. In an embodiment, the vertically-oriented bit line select transistors are formed in a pillar select layer formed above a CMOS substrate, and a memory layer that includes multiple layers of word lines and memory elements is formed above the pillar select layer.
FIGS. 5A-5D depict various views of an embodiment of a portion of a monolithic three-dimensional memory array 500 that includes vertical strips of a non-volatile memory material. The physical structure depicted in FIGS. 5A-5D may include one implementation for a portion of the monolithic three-dimensional memory array depicted in FIG. 4A.
Monolithic three-dimensional memory array 500 includes vertical bit lines LBL11-LBL33 arranged in a first direction (e.g., a z-direction), word lines WL10, WL11, . . . , WL43 arranged in a second direction (e.g., an x-direction) perpendicular to the first direction, row select lines SG1, SG2, SG3 arranged in the second direction, and global bit lines GBL1, GBL2, GBL3 arranged in a third direction (e.g., a y-direction) perpendicular to the first and second directions. Vertical bit lines LBL11-LBL33 are disposed above global bit lines GBL1, GBL2, GBL3, which each have a long axis in the second (e.g., x-direction). Person of ordinary skill in the art will understand that monolithic three-dimensional memory arrays, such as monolithic three-dimensional memory array 500 may include more or fewer than twenty-four word lines, three row select lines, three global bit lines, and nine vertical bit lines.
In an embodiment, global bit lines GBL1, GBL2, GBL3 are disposed above a substrate 502, such as a silicon, germanium, silicon-germanium, undoped, doped, bulk, silicon-on-insulator (“SOI”) or other substrate with or without additional circuitry. In an embodiment, an isolation layer 504, such as a layer of silicon dioxide, silicon nitride, silicon oxynitride or any other suitable insulating layer, is formed above substrate 502.
In an embodiment, a first dielectric material layer 508 (e.g., silicon dioxide) and a second dielectric material layer 510 (e.g., silicon dioxide) are formed above isolation layer 504. Global bit lines GBL1, GBL2, GBL3 are disposed above isolation layer 504 and are separated by first dielectric material layer 508. Row select lines SG1, SG2, SG3 are disposed above global bit lines GBL1, GBL2, GBL3. A first etch stop layer 512 (e.g., silicon nitride) is disposed above second dielectric material layer 510. A stack of word lines WL10, WL11, . . . , WL43 is disposed above first etch stop layer 512, with a third dielectric material layer 514 (e.g., silicon dioxide) separating adjacent word lines. A second etch stop layer 516 (e.g., silicon nitride) is disposed above the stack of word lines WL10, WL11, . . . , WL43.
In an embodiment, vertical strips of a non-volatile memory material 518 are disposed adjacent word lines WL10, WL11, . . . , WL43. The vertical strips of non-volatile memory material 518 may be formed in the first direction (e.g., the z-direction). A vertical strip of non-volatile memory material 518 may include, for example, a vertical oxide layer, a vertical reversible resistance-switching material (e.g., a metal oxide material, a phase change material or other similar reversible resistance-switching material), a vertical charge trapping layer (e.g., a layer of silicon nitride), or other non-volatile memory material. A vertical strip of non-volatile memory material 518 may include a single continuous layer of material that may be used by a plurality of memory cells or devices. In an embodiment, vertical strip of non-volatile memory material 518 includes one or more of HfO2, Al2O3, HfSiOx, HfSiOxNy, HfAlOx, Nb2O5, Ta2O5, ZrO2, Cr2O3, Fe2O3, Ni2O3, Co2O3, WO3, TiO2, SrZrO3, SrTiO3.
Vertical bit lines LBL11-LBL33 are formed of a conductive material (e.g., a highly doped polysilicon material). Vertical bit lines LBL11-LBL33 also may include an adhesion layer 520 (e.g., titanium nitride) disposed on an outer surface of each vertical bit line LBL11-LBL33. In some embodiments, each of a vertical bit lines LBL11-LBL33 includes a vertical structure (e.g., a rectangular prism, a cylinder, or a pillar), and the vertical strip of non-volatile memory material 518 may completely or partially surround the vertical structure (e.g., a conformal layer of reversible resistance-switching material surrounding the sides of the vertical structure).
A memory cell is disposed between the intersection of each vertical bit line and each word line. For example, a memory cell M111 is disposed between vertical bit line LBL11 and word line WL10, a memory cell M116 is disposed between vertical bit line LBL13 and word line WL13, a memory cell M411 is disposed between vertical bit line LBL11 and word line WL40, a memory cell M432 is disposed between vertical bit line LBL31 and word line WL41, and a memory cell M436 is disposed between vertical bit line LBL33 and word line WL43. In an embodiment, monolithic three-dimensional memory array 500 includes seventy-two memory cells M111, M112, . . . , M436. Persons of ordinary skill in the art will understand that monolithic three-dimensional memory arrays may include more or fewer than seventy-two memory cells.
In one example, portions of the vertical strip of non-volatile memory material 518 may include a part of memory cell M111 associated with the cross section between WL10 and LBL11, and a part of memory cell M211 associated with the cross section between WL20 and LBL11.
Each of memory cells M111, M112, . . . , M436 may include a floating gate device, a charge trap device (e.g., using a silicon nitride material), a resistive change memory device, or other type of memory device. Vertically-oriented bit line select transistors Q11-Q33 may be used to select a corresponding one of vertical bit lines LBL11-LBL33. Vertically-oriented bit line select transistors Q11-Q33 may be field effect transistors, although other transistors types may be used.
Each of vertically-oriented bit line select transistors Q11-Q33 has a first terminal (e.g., a drain/source terminal), a second terminal (e.g., a source/drain terminal), and a third terminal (e.g., a gate terminal). The gate terminal may be disposed on all sides of the vertically-oriented bit line select transistor, and is also referred to herein as a “surround gate.” The surround gate may be used to selectively induce a conductive channel between the first terminal and the second terminal of the transistor to turn ON and OFF the vertically-oriented bit line select transistor.
In an embodiment, the surround gates of vertically-oriented bit line select transistors Q11, Q21 and Q31 are coupled to row select line SG1, the surround gates of vertically-oriented bit line select transistors Q12, Q22 and Q32 are coupled to row select line SG2, and the surround gates of vertically-oriented bit line select transistors Q13, Q23 and Q33 are coupled to row select line SG3. Other configurations also may be used.
Thus, row select line SG1 and vertically-oriented bit line select transistors Q11, Q21 and Q31 may be used to select vertical bit lines LBL11, LBL21 and LBL31, row select line SG2 and vertically-oriented bit line select transistors Q12, Q22 and Q32 may be used to select vertical bit lines LBL12, LBL22 and LBL32, and row select line SG3 and vertically-oriented bit line select transistors Q13, Q23 and Q33 may be used to select vertical bit lines LBL13, LBL23 and LBL33. Without wanting to be bound by any particular theory, for each of vertically-oriented bit line select transistors Q11-Q33, it is believed that the current drive capability of the transistor may be increased by using a surround gate to turn ON the transistor.
Referring to FIGS. 5A and 5D, vertically-oriented bit line select transistors Q11, Q12, Q13 are used to selectively connect/disconnect vertical bit lines LBL11, LBL12, LBL13 to/from global bit line GBL1 using row select lines SG1, SG2, SG3, respectively. In particular, each of vertically-oriented bit line select transistors Q11, Q12, Q13 has a first terminal (e.g., a drain/source terminal) coupled to a corresponding one of vertical bit lines LBL11, LBL12, LBL13, respectively, a second terminal (e.g., a source/drain terminal) coupled to global bit line GBL1, and a third terminal (e.g., a surround gate) coupled to row select line SG1, SG2, SG3, respectively.
Row select lines SG1, SG2, SG3 are used to turn ON/OFF vertically-oriented bit line select transistors Q11, Q12, Q13, respectively, to connect/disconnect vertical bit lines LBL11, LBL12, LBL13, respectively, to/from global bit line GBL1. A gate dielectric material layer 522 (e.g., silicon dioxide) is disposed between row select lines SG2, SG3 and vertically-oriented bit line select transistors Q11, Q12, Q13.
Likewise, referring to FIGS. 5A-5D, vertically-oriented bit line select transistors Q11, Q21, . . . , Q33 are used to selectively connect/disconnect vertical bit lines LBL11, LBL21, LBL31 to global bit lines GBL1, GBL2, GBL3, respectively, using row select line SG1. In particular, each of vertically-oriented bit line select transistors Q11, Q21, Q31 has a first terminal (e.g., a drain./source terminal) coupled to a corresponding one of vertical bit lines LBL11, LBL21, LBL31, respectively, a second terminal (e.g., a source/drain terminal) coupled to a corresponding one of global bit lines GBL1, GBL2, GBL3, respectively, and a control terminal (e.g., a surround gate) coupled to row select line SG1. Row select line SG1 is used to turn ON/OFF vertically-oriented bit line select transistors Q11, Q21, Q31 to connect/disconnect vertical bit lines LBL11, LBL21, LBL31, respectively, to/from global bit lines GBL1, GBL2, GBL3, respectively.
Similarly, vertically-oriented bit line select transistors Q13, Q23, Q33 are used to selectively connect/disconnect vertical bit lines LBL13, LBL23, LBL33 to/from global bit lines GBL1, GBL2, GBL3, respectively, using row select line SG3. In particular, each of vertically-oriented bit line select transistors Q13, Q23, Q33 has a first terminal (e.g., a drain./source terminal) coupled to a corresponding one of vertical bit lines LBL13, LBL23, LBL33, respectively, a second terminal (e.g., a source/drain terminal) coupled to a corresponding one of global bit lines GBL1, GBL2, GBL3, respectively, and a control terminal (e.g., a surround gate) coupled to row select line SG3. Row select line SG3 is used to turn ON/OFF vertically-oriented bit line select transistors Q13, Q23, Q33 to connect/disconnect vertical bit lines LBL13, LBL23, LBL33, respectively, to/from global bit lines GBL1, GBL2, GBL3, respectively.
Referring now to FIGS. 6A1-6L2, an example method of forming a portion of a monolithic three-dimensional memory array, such as monolithic three-dimensional array 500 of FIGS. 5A-5D, is described.
With reference to FIGS. 6A1-6A3, substrate 502 is shown as having already undergone several processing steps. Substrate 502 may be any suitable substrate such as a silicon, germanium, silicon-germanium, undoped, doped, bulk, SOI or other substrate with or without additional circuitry. For example, substrate 502 may include one or more n-well or p-well regions (not shown). Isolation layer 504 is formed above substrate 502. In some embodiments, isolation layer 504 may be a layer of silicon dioxide, silicon nitride, silicon oxynitride or any other suitable insulating layer.
Following formation of isolation layer 504, a conductive material layer 506 is deposited over isolation layer 504. Conductive material layer 506 may include any suitable conductive material such as tungsten or another appropriate metal, heavily doped semiconductor material, a conductive silicide, a conductive silicide-germanide, a conductive germanide, or the like deposited by any suitable method (e.g., CVD, PVD, etc.). In at least one embodiment, conductive material layer 506 may comprise between about 200 and about 2500 angstroms of tungsten. Other conductive material layers and/or thicknesses may be used. In some embodiments, an adhesion layer (not shown), such as titanium nitride or other similar adhesion layer material, may be disposed between isolation layer 504 and conductive material layer 506, and/or between conductive material layer 506 and subsequent vertically-oriented bit line select transistors layers.
Persons of ordinary skill in the art will understand that adhesion layers may be formed by PVD or another method on conductive material layers. For example, adhesion layers may be between about 20 and about 500 angstroms, and in some embodiments about 100 angstroms, of titanium nitride or another suitable adhesion layer such as tantalum nitride, tungsten nitride, tungsten, molybdenum, combinations of one or more adhesion layers, or the like. Other adhesion layer materials and/or thicknesses may be employed.
Following formation of conductive material layer 506, conductive material layer 506 is patterned and etched. For example, conductive material layer 506 may be patterned and etched using conventional lithography techniques, with a soft or hard mask, and wet or dry etch processing. In at least one embodiment, conductive material layer 506 is patterned and etched to form global bit lines GBL1, GBL2, GBL3. Example widths for global bit lines GBL1, GBL2, GBL3 and/or spacings between global bit lines GBL1, GBL2, GBL3 range between about 480 angstroms and about 1000 angstroms, although other conductor widths and/or spacings may be used.
After global bit lines GBL1, GBL2, GBL3 have been formed, a first dielectric material layer 508 is formed over substrate 502 to fill the voids between global bit lines GBL1, GBL2, GBL3. For example, approximately 3000-7000 angstroms of silicon dioxide may be deposited on the substrate 502 and planarized using chemical mechanical polishing or an etchback process to form a planar surface 600. Other dielectric materials such as silicon nitride, silicon oxynitride, low K dielectrics, etc., and/or other dielectric material layer thicknesses may be used. Example low K dielectrics include carbon doped oxides, silicon carbon layers, or the like.
In other embodiments, global bit lines GBL1, GBL2, GBL3 may be formed using a damascene process in which first dielectric material layer 508 is formed, patterned and etched to create openings or voids for global bit lines GBL1, GBL2, GBL3. The openings or voids then may be filled with conductive layer 506 (and/or a conductive seed, conductive fill and/or barrier layer if needed). Conductive material layer 506 then may be planarized to form planar surface 600.
Following planarization, the semiconductor material used to form vertically-oriented bit line select transistors Q11-Q33 is formed over planarized top surface 600 of substrate 502. In some embodiments, each vertically-oriented bit line select transistor is formed from a polycrystalline semiconductor material such as polysilicon, an epitaxial growth silicon, a polycrystalline silicon-germanium alloy, polygermanium or any other suitable material. Alternatively, vertically-oriented bit line select transistors Q11-Q33 may be formed from a wide band-gap semiconductor material, such as ZnO, InGaZnO, or SiC, which may provide a high breakdown voltage, and typically may be used to provide junctionless FETs. Persons of ordinary skill in the art will understand that other materials may be used.
In some embodiments, each vertically-oriented bit line select transistor Q11-Q33 may include a first region (e.g., n+ polysilicon), a second region (e.g., p polysilicon) and a third region (e.g., n+ polysilicon) to form drain/source, body, and source/drain regions, respectively, of a vertical FET. For example, a heavily doped n+ polysilicon layer 602 may be deposited on planarized top surface 600. In some embodiments, n+ polysilicon layer 602 is in an amorphous state as deposited. In other embodiments, n+ polysilicon layer 602 is in a polycrystalline state as deposited. CVD or another suitable process may be employed to deposit n+ polysilicon layer 602.
In an embodiment, n+ polysilicon layer 602 may be formed, for example, from about 100 to about 500 angstroms, of phosphorus or arsenic doped silicon having a doping concentration of about 1021 cm−3. Other layer thicknesses, doping types and/or doping concentrations may be used. N+ polysilicon layer 602 may be doped in situ, for example, by flowing a donor gas during deposition. Other doping methods may be used (e.g., implantation).
After deposition of n+ silicon layer 602, a doped p-type silicon layer 604 may be formed over n+ polysilicon layer 602. P-type silicon may be either deposited and doped by ion implantation or may be doped in situ during deposition to form a p-type silicon layer 604. For example, an intrinsic silicon layer may be deposited on n+ polysilicon layer 602, and a blanket p-type implant may be employed to implant boron a predetermined depth within the intrinsic silicon layer. Example implantable molecular ions include BF2, BF3, B and the like. In some embodiments, an implant dose of about 1-10×1013 ions/cm2 may be employed. Other implant species and/or doses may be used. Further, in some embodiments, a diffusion process may be employed. In an embodiment, the resultant p-type silicon layer 604 has a thickness of from about 800 to about 4000 angstroms, although other p-type silicon layer sizes may be used.
Following formation of p-type silicon layer 604, a heavily doped n+ polysilicon layer 606 is deposited on p-type silicon layer 604. In some embodiments, n+ polysilicon layer 606 is in an amorphous state as deposited. In other embodiments, n+ polysilicon layer 606 is in a polycrystalline state as deposited. CVD or another suitable process may be employed to deposit n+ polysilicon layer 606.
In an embodiment, n+ polysilicon layer 606 may be formed, for example, from about 100 to about 500 angstroms of phosphorus or arsenic doped silicon having a doping concentration of about 1021 cm−3. Other layer thicknesses, doping types and/or doping concentrations may be used. N+ polysilicon layer 606 may be doped in situ, for example, by flowing a donor gas during deposition. Other doping methods may be used (e.g., implantation). Persons of ordinary skill in the art will understand that silicon layers 602, 604 and 606 alternatively may be doped p+/n/p+, respectively, or may be doped with a single type of dopant to produce junctionless-FETs.
Following formation of n+ polysilicon layer 606, a hard mask material layer 608 is deposited on n+ polysilicon layer 606, resulting in the structure shown in FIGS. 6B1-6B3. Hard mask material layer 608 may include any suitable hard mask material such as silicon nitride, amorphous carbon, or the like deposited by any suitable method (e.g., CVD, PVD, etc.). In at least one embodiment, hard mask material layer 608 may comprise between about 300 angstroms and about 800 angstroms of silicon nitride. Other hard mask materials and/or thicknesses may be used.
Hard mask material layer 608 and silicon layers 602, 604 and 606 are patterned and etched to form rows 610 of silicon layers 602, 604 and 606 and hard mask material layer 608, resulting in the structure shown in FIGS. 6C1-6C3. For example, hard mask material layer 608 and silicon layers 602, 604 and 606 may be patterned and etched using conventional lithography techniques, with wet or dry etch processing.
Hard mask material layer 608 and silicon layers 602, 604 and 606 may be patterned and etched in a single pattern/etch procedure or using separate pattern/etch steps. Any suitable masking and etching process may be used to form vertical transistor pillars. For example, silicon layers may be patterned with about 1 to about 1.5 micron, more preferably about 1.2 to about 1.4 micron, of photoresist (“PR”) using standard photolithographic techniques. Thinner PR layers may be used with smaller critical dimensions and technology nodes. In some embodiments, an oxide hard mask may be used below the PR layer to improve pattern transfer and protect underlying layers during etching.
In some embodiments, after etching, rows 610 may be cleaned using a dilute hydrofluoric/sulfuric acid clean. Such cleaning may be performed in any suitable cleaning tool, such as a Raider tool, available from Semitool of Kalispell, Mont. Example post-etch cleaning may include using ultra-dilute sulfuric acid (e.g., about 1.5 1.8 wt %) for about 60 seconds and/or ultra-dilute hydrofluoric (“HF”) acid (e.g., about 0.4-0.6 wt %) for 60 seconds. Megasonics may or may not be used. Other clean chemistries, times and/or techniques may be employed.
A first gate dielectric material layer 522a is deposited conformally over substrate 502, and forms on sidewalls of rows 610. For example, between about 30 angstroms to about 100 angstroms of silicon dioxide may be deposited. Other dielectric materials such as silicon nitride, silicon oxynitride, low K dielectrics, etc., and/or other dielectric material layer thicknesses may be used.
An anisotropic etch is used to remove lateral portions of first gate dielectric material, leaving only sidewall portions of first gate dielectric material layer 522a, resulting in the structure shown in FIGS. 6D1-6D3.
A first gate electrode material is deposited over rows 610 and first gate dielectric material layer 522a. For example, approximately 50 angstroms to about 500 angstroms of a highly-doped semiconductor, such as n+ polysilicon, p+ polysilicon, or other similar conductive material may be deposited. Other conductive materials and/or thicknesses may be used. The as-deposited gate electrode material is subsequently etched back to form first rail gates 612a, resulting in the structure shown in FIGS. 6E1-6E3. First rail gate 612a are disposed on first sides of the vertical stack of silicon layers 602, 604 and 606. Other conductive materials and/or thicknesses may be used for first rail gates 612a. 
A first liner material layer 614 is deposited conformally over substrate 502, and forms on sidewalls of rows 610. For example, between about 30 angstroms to about 100 angstroms of silicon dioxide may be deposited. Other dielectric materials such as silicon nitride, silicon oxynitride, low K dielectrics, etc., and/or other dielectric material layer thicknesses may be used. An anisotropic etch is used to remove lateral portions of liner material, leaving only sidewall portions of first liner material layer 614.
A sacrificial material layer 616 is deposited over substrate 602, filling voids between first liner material layers 614. For example, approximately 200 angstroms to about 500 angstroms of polysilicon 616 may be deposited and planarized using chemical mechanical polishing or an etch-back process to form planar top surface 618, resulting in the structure shown in FIGS. 6F1-6F4. Other sacrificial materials and thicknesses may be used.
Following formation of sacrificial material layer 616, sacrificial material layer 616, first liner material layers 614, first rail gates 612a, hard mask material layer 608, first gate dielectric material layer 522a, and silicon layers 602, 604 and 606 are patterned and etched to form vertical transistor pillars, resulting in the structure shown in FIGS. 6G1-6G4. For example, sacrificial material layer 616, sacrificial material layer 616, first liner material layers 614, hard mask material layer 608, first rail gates 612a, first gate dielectric material layer 522a, and silicon layers 602, 604 and 606 may be patterned and etched using conventional lithography techniques, with wet or dry etch processing.
In an embodiment, sacrificial material layer 616, first liner material layers 614, first rail gates 612a, hard mask material layer 608, first gate dielectric material layer 522a, and silicon layers 602, 604 and 606 are patterned and etched to form vertical transistor pillars disposed above global bit lines GBL1, GBL2, GBL3. The vertical transistor pillars will be used to form vertically-oriented bit line select transistors Q11-Q33.
A second gate dielectric material layer 522b is deposited conformally over substrate 502, and forms on sidewalls of silicon layers 602, 604 and 606. For example, between about 30 angstroms to about 100 angstroms of silicon dioxide may be deposited. Other dielectric materials such as silicon nitride, silicon oxynitride, low K dielectrics, etc., and/or other dielectric material layer thicknesses may be used.
An anisotropic etch is used to remove lateral portions of second gate dielectric material, leaving only sidewall portions of second gate dielectric material layer 522b, resulting in the structure shown in FIGS. 6H1-6H4.
A second gate electrode material is deposited over second gate dielectric material layer 522b. For example, approximately 50 angstroms to about 500 angstroms of a highly-doped semiconductor, such as n+ polysilicon, p+ polysilicon, or other similar conductive material may be deposited. The as-deposited gate electrode material is subsequently etched back to form second rail gates 612b, resulting in the structure shown in FIGS. 6I1-6I4. Second rail gate 612b are disposed on second sides of the vertical stack of silicon layers 602, 604 and 606. Other conductive materials and/or thicknesses may be used for second rail gates 612b. In addition, first rail gates 612a and second rail gates 612b may be the same material or may be different materials.
A second liner material layer 620 is deposited conformally over substrate 502, and forms on sidewalls of second rail gates 612b. For example, between about 30 angstroms to about 100 angstroms of silicon nitride may be deposited. Other dielectric materials such as silicon dioxide, silicon oxynitride, low K dielectrics, etc., and/or other dielectric material layer thicknesses may be used. An anisotropic etch is used to remove lateral portions of liner material, leaving only sidewall portions of second liner material layer 620.
A second dielectric material layer 510 is deposited over substrate 502, filling voids between second liner material layer 620. For example, approximately 5000 to about 8000 angstroms of silicon dioxide may be deposited and planarized using chemical mechanical polishing or an etch-back process to form planar top surface 622, resulting in the structure shown in FIGS. 6J1-6J2.
Next, first liner material layers 614, sacrificial material layers 616 and portions of second gate dielectric material layers 522b adjacent to sacrificial material layers 616 are removed, for example by a wet or dry etch process, resulting in the structure shown in FIGS. 6K1-6K2.
A third gate electrode material is deposited over substrate 502. For example, approximately 50 angstroms to about 1000 angstroms of a highly-doped semiconductor, such as n+ polysilicon, p+ polysilicon, or other similar conductive material may be deposited. The as-deposited gate electrode material is subsequently etched back to the same height as first rail gates 612a and second rail gates 612b to form bridge sections 612c, resulting in the structure shown in FIGS. 6L1-6L2. Other conductive materials and/or thicknesses may be used for bridge sections 612c. First rail gates 612a, second rail gates 612b and bridge sections 612c collectively form row select lines SG1, SG2, SG3. First rail gates 612a, second rail gates 612b and bridge sections 612c may be the same material or may be different materials.
Thus, the example process steps described above and illustrated in FIGS. 6A1-6L2 form vertically-oriented bit line select transistors Q11-Q33 that include surround gates formed by first rail gates 612a, second rail gates 612b and bridge sections 612c coupled between first rail gates 612a and second rail gates 612b, and disposed on all sides of the vertically-oriented bit line select transistors.
In particular, the surround gates of vertically-oriented bit line select transistors Q11, Q21 and Q31 are coupled to row select line SG1, the surround gates of vertically-oriented bit line select transistors Q12, Q22 and Q32 are coupled to row select line SG2, and the surround gates of vertically-oriented bit line select transistors Q13, Q23 and Q33 are coupled to row select line SG3.
Following the example process illustrated in FIGS. 6A1-6L2, additional process steps may be used to form word lines WL10, WL11, . . . , WL43 arranged and vertical bit lines LBL11-LBL33 above vertically-oriented bit line select transistors Q11-Q33 and global bit lines GBL1-GBL3.
FIGS. 7A-7E depict various views of an embodiment of a portion of a monolithic three-dimensional memory array 700 that includes vertical strips of a non-volatile memory material. The physical structure depicted in FIGS. 7A-7E may include one implementation for a portion of the monolithic three-dimensional memory array depicted in FIG. 4A.
Monolithic three-dimensional memory array 700 includes vertical bit lines LBL11-LBL33 arranged in a first direction (e.g., a z-direction), word lines WL10, WL11, . . . , WL43 arranged in a second direction (e.g., an x-direction) perpendicular to the first direction, row select lines SG1, SG2, SG3 arranged in the second direction, and global bit lines GBL1, GBL2, GBL3 arranged in a third direction (e.g., a y-direction) perpendicular to the first and second directions. Vertical bit lines LBL11-LBL33 are disposed above global bit lines GBL1, GBL2, GBL3, which each have a long axis in the second (e.g., x-direction). Person of ordinary skill in the art will understand that monolithic three-dimensional memory arrays, such as monolithic three-dimensional memory array 700 may include more or fewer than twenty-four word lines, three row select lines, three global bit lines, and nine vertical bit lines.
In an embodiment, global bit lines GBL1, GBL2, GBL3 are disposed above a substrate 502, such as a silicon, germanium, silicon-germanium, undoped, doped, bulk, SOI or other substrate with or without additional circuitry. In an embodiment, an isolation layer 504, such as a layer of silicon dioxide, silicon nitride, silicon oxynitride or any other suitable insulating layer, is formed above substrate 502.
In an embodiment, a first dielectric material layer 508 (e.g., silicon dioxide) and a second dielectric material layer 510 (e.g., silicon dioxide) are formed above isolation layer 504. Global bit lines GBL1, GBL2, GBL3 are disposed above isolation layer 504 and are separated by first dielectric material layer 508. Row select lines SG1, SG2, SG3 are disposed above global bit lines GBL1, GBL2, GBL3. A first etch stop layer 512 (e.g., silicon nitride) is disposed above second dielectric material layer 510. A stack of word lines WL10, WL11, . . . , WL43 is disposed above first etch stop layer 512, with a third dielectric material layer 514 (e.g., silicon dioxide) separating adjacent word lines. A second etch stop layer 516 (e.g., silicon nitride) is disposed above the stack of word lines WL10, WL11, . . . , WL43.
In an embodiment, vertical strips of a non-volatile memory material 518 are disposed adjacent word lines WL10, WL11, . . . , WL43. The vertical strips of non-volatile memory material 518 may be formed in the first direction (e.g., the z-direction). A vertical strip of non-volatile memory material 518 may include, for example, a vertical oxide layer, a vertical reversible resistance-switching material (e.g., a metal oxide material, a phase change material or other similar reversible resistance-switching material), a vertical charge trapping layer (e.g., a layer of silicon nitride), or other non-volatile memory material. A vertical strip of non-volatile memory material 518 may include a single continuous layer of material that may be used by a plurality of memory cells or devices. In an embodiment, vertical strip of non-volatile memory material 518 includes one or more of HfO2, Al2O3, HfSiOx, HfSiOxNy, HfAlOx, Nb2O5, Ta2O5, ZrO2, Cr2O3, Fe2O3, Ni2O3, Co2O3, WO3, TiO2, SrZrO3, SrTiO3.
Vertical bit lines LBL11-LBL33 are formed of a conductive material (e.g., a highly doped polysilicon material). Vertical bit lines LBL11-LBL33 also may include an adhesion layer 520 (e.g., titanium nitride) disposed on an outer surface of each vertical bit line LBL11-LBL33. In some embodiments, each of a vertical bit lines LBL11-LBL33 includes a vertical structure (e.g., a rectangular prism, a cylinder, or a pillar), and the vertical strip of non-volatile memory material 518 may completely or partially surround the vertical structure (e.g., a conformal layer of reversible resistance-switching material surrounding the sides of the vertical structure).
A memory cell is disposed between the intersection of each vertical bit line and each word line. For example, a memory cell M111 is disposed between vertical bit line LBL11 and word line WL10, a memory cell M116 is disposed between vertical bit line LBL13 and word line WL13, a memory cell M411 is disposed between vertical bit line LBL11 and word line WL40, a memory cell M432 is disposed between vertical bit line LBL31 and word line WL41, and a memory cell M436 is disposed between vertical bit line LBL33 and word line WL43. In an embodiment, monolithic three-dimensional memory array 500 includes seventy-two memory cells M111, M112, . . . , M436. Persons of ordinary skill in the art will understand that monolithic three-dimensional memory arrays may include more or fewer than seventy-two memory cells.
In one example, portions of the vertical strip of non-volatile memory material 518 may include a part of memory cell M111 associated with the cross section between WL10 and LBL11, and a part of memory cell M211 associated with the cross section between WL20 and LBL11.
Each of memory cells M111, M112, . . . , M436 may include a floating gate device, a charge trap device (e.g., using a silicon nitride material), a resistive change memory device, or other type of memory device. Vertically-oriented bit line select transistors Q11-Q33 may be used to select a corresponding one of vertical bit lines LBL11-LBL33. Vertically-oriented bit line select transistors Q11-Q33 may be field effect transistors, although other transistors types may be used.
Each of vertically-oriented bit line select transistors Q11-Q33 has a first terminal (e.g., a drain/source terminal), a second terminal (e.g., a source/drain terminal), and a third terminal (e.g., a gate terminal). The gate terminal may be a surround gate that may be used to selectively induce a conductive channel between the first terminal and the second terminal of the transistor to turn ON and OFF the vertically-oriented bit line select transistor.
In an embodiment, the surround gates of vertically-oriented bit line select transistors Q11, Q21 and Q31 are coupled to row select line SG1, the surround gates of vertically-oriented bit line select transistors Q12, Q22 and Q32 are coupled to row select line SG2, and the surround gates of vertically-oriented bit line select transistors Q13, Q23 and Q33 are coupled to row select line SG3. Other configurations also may be used.
Thus, row select line SG1 and vertically-oriented bit line select transistors Q11, Q21 and Q31 may be used to select vertical bit lines LBL11, LBL21 and LBL31, row select line SG2 and vertically-oriented bit line select transistors Q12, Q22 and Q32 may be used to select vertical bit lines LBL12, LBL22 and LBL32, and row select line SG3 and vertically-oriented bit line select transistors Q13, Q23 and Q33 may be used to select vertical bit lines LBL13, LBL23 and LBL33. Without wanting to be bound by any particular theory, for each of vertically-oriented bit line select transistors Q11-Q33, it is believed that the current drive capability of the transistor may be increased by using a surround gate to turn ON the transistor.
Referring to FIGS. 7A and 7E, vertically-oriented bit line select transistors Q11, Q12, Q13 are used to selectively connect/disconnect vertical bit lines LBL11, LBL12, LBL13 to/from global bit line GBL1 using row select lines SG1, SG2, SG3, respectively. In particular, each of vertically-oriented bit line select transistors Q11, Q12, Q13 has a first terminal (e.g., a drain./source terminal) coupled to a corresponding one of vertical bit lines LBL11, LBL12, LBL13, respectively, a second terminal (e.g., a source/drain terminal) coupled to global bit line GBL1, and a third terminal (e.g., a surround gate) coupled to row select line SG1, SG2, SG3, respectively.
Row select lines SG1, SG2, SG3 are used to turn ON/OFF vertically-oriented bit line select transistors Q11, Q12, Q13, respectively, to connect/disconnect vertical bit lines LBL11, LBL12, LBL13, respectively, to/from global bit line GBL1. A gate dielectric material layer 522 (e.g., silicon dioxide) is disposed between row select lines SG2, SG3 and vertically-oriented bit line select transistors Q11, Q12, Q13.
Likewise, referring to FIGS. 7A-7E, vertically-oriented bit line select transistors Q11, Q21, . . . , Q33 are used to selectively connect/disconnect vertical bit lines LBL11, LBL21, LBL31 to global bit lines GBL1, GBL2, GBL3, respectively, using row select line SG1. In particular, each of vertically-oriented bit line select transistors Q11, Q21, Q31 has a first terminal (e.g., a drain./source terminal) coupled to a corresponding one of vertical bit lines LBL11, LBL21, LBL31, respectively, a second terminal (e.g., a source/drain terminal) coupled to a corresponding one of global bit lines GBL1, GBL2, GBL3, respectively, and a control terminal (e.g., a surround gate) coupled to row select line SG1. Row select line SG1 is used to turn ON/OFF vertically-oriented bit line select transistors Q11, Q21, Q31 to connect/disconnect vertical bit lines LBL11, LBL21, LBL31, respectively, to/from global bit lines GBL1, GBL2, GBL3, respectively.
Similarly, vertically-oriented bit line select transistors Q13, Q23, Q33 are used to selectively connect/disconnect vertical bit lines LBL13, LBL23, LBL33 to/from global bit lines GBL1, GBL2, GBL3, respectively, using row select line SG3. In particular, each of vertically-oriented bit line select transistors Q13, Q23, Q33 has a first terminal (e.g., a drain./source terminal) coupled to a corresponding one of vertical bit lines LBL13, LBL23, LBL33, respectively, a second terminal (e.g., a source/drain terminal) coupled to a corresponding one of global bit lines GBL1, GBL2, GBL3, respectively, and a control terminal (e.g., a surround gate) coupled to row select line SG3. Row select line SG3 is used to turn ON/OFF vertically-oriented bit line select transistors Q13, Q23, Q33 to connect/disconnect vertical bit lines LBL13, LBL23, LBL33, respectively, to/from global bit lines GBL1, GBL2, GBL3, respectively.
Each of row select lines SG1, SG2 and SG3 includes first and second bridge portions extending in a direction along a long axis (e.g., along the y-axis in FIGS. 7B and 7C) of row select lines SG1, SG2 and SG3. For example, referring to FIGS. 7B-7D, each of row select lines SG1, SG2 and SG3 includes first bridge portions 524a disposed near a top of row select lines SG1, SG2 and SG3, and second bridge portions 524b disposed near a bottom of row select lines SG1, SG2 and SG3. A cavity 526 is disposed between each of first bridge portions 524a and second bridge portions 524b. Each cavity 526 may be filled with a dielectric material (e.g., silicon dioxide). Without wanting to be bound by any particular theory, it is believed that by virtue of having two bride portions, first bridge portions 524a and second bridge portions 524b may lower a resistance of row select lines SG1, SG2 and SG3.
Referring now to FIGS. 8A1-8J5, an example method of forming a portion of a monolithic three-dimensional memory array, such as monolithic three-dimensional array 700 of FIGS. 7A-7E, is described.
With reference to FIGS. 8A1-8A3, substrate 502 is shown as having already undergone several processing steps. Substrate 502 may be any suitable substrate such as a silicon, germanium, silicon-germanium, undoped, doped, bulk, SOI or other substrate with or without additional circuitry. For example, substrate 502 may include one or more n-well or p-well regions (not shown). Isolation layer 504 is formed above substrate 502. In some embodiments, isolation layer 504 may be a layer of silicon dioxide, silicon nitride, silicon oxynitride or any other suitable insulating layer.
Following formation of isolation layer 504, a conductive material layer 506 is deposited over isolation layer 504. Conductive material layer 506 may include any suitable conductive material such as tungsten or another appropriate metal, heavily doped semiconductor material, a conductive silicide, a conductive silicide-germanide, a conductive germanide, or the like deposited by any suitable method (e.g., CVD, PVD, etc.). In at least one embodiment, conductive material layer 506 may comprise between about 200 and about 2500 angstroms of tungsten. Other conductive material layers and/or thicknesses may be used. In some embodiments, an adhesion layer (not shown), such as titanium nitride or other similar adhesion layer material, may be disposed between isolation layer 504 and conductive material layer 506, and/or between conductive material layer 506 and subsequent vertically-oriented bit line select transistors layers.
Persons of ordinary skill in the art will understand that adhesion layers may be formed by PVD or another method on conductive material layers. For example, adhesion layers may be between about 20 and about 500 angstroms, and in some embodiments about 100 angstroms, of titanium nitride or another suitable adhesion layer such as tantalum nitride, tungsten nitride, tungsten, molybdenum, combinations of one or more adhesion layers, or the like. Other adhesion layer materials and/or thicknesses may be employed.
Following formation of conductive material layer 506, conductive material layer 506 is patterned and etched. For example, conductive material layer 506 may be patterned and etched using conventional lithography techniques, with a soft or hard mask, and wet or dry etch processing. In at least one embodiment, conductive material layer 506 is patterned and etched to form global bit lines GBL1, GBL2, GBL3. Example widths for global bit lines GBL1, GBL2, GBL3 and/or spacings between global bit lines GBL1, GBL2, GBL3 range between about 480 angstroms and about 1000 angstroms, although other conductor widths and/or spacings may be used.
After global bit lines GBL1, GBL2, GBL3 have been formed, a first dielectric material layer 508 is formed over substrate 502 to fill the voids between global bit lines GBL1, GBL2, GBL3. For example, approximately 3000-7000 angstroms of silicon dioxide may be deposited on the substrate 502 and planarized using chemical mechanical polishing or an etchback process to form a planar surface 800. Other dielectric materials such as silicon nitride, silicon oxynitride, low K dielectrics, etc., and/or other dielectric material layer thicknesses may be used. Example low K dielectrics include carbon doped oxides, silicon carbon layers, or the like.
In other embodiments, global bit lines GBL1, GBL2, GBL3 may be formed using a damascene process in which first dielectric material layer 508 is formed, patterned and etched to create openings or voids for global bit lines GBL1, GBL2, GBL3. The openings or voids then may be filled with conductive layer 506 (and/or a conductive seed, conductive fill and/or barrier layer if needed). Conductive material layer 506 then may be planarized to form planar surface 800.
Following planarization, the semiconductor material used to form vertically-oriented bit line select transistors Q11-Q33 is formed over planarized top surface 800 of substrate 502. In some embodiments, each vertically-oriented bit line select transistor is formed from a polycrystalline semiconductor material such as polysilicon, an epitaxial growth silicon, a polycrystalline silicon-germanium alloy, polygermanium or any other suitable material. Alternatively, vertically-oriented bit line select transistors Q11-Q33 may be formed from a wide band-gap semiconductor material, such as ZnO, InGaZnO, or SiC, which may provide a high breakdown voltage, and typically may be used to provide junctionless FETs. Persons of ordinary skill in the art will understand that other materials may be used.
In some embodiments, each vertically-oriented bit line select transistor Q11-Q33 may include a first region (e.g., n+ polysilicon), a second region (e.g., p polysilicon) and a third region (e.g., n+ polysilicon) to form drain/source, body, and source/drain regions, respectively, of a vertical FET. For example, a heavily doped n+ polysilicon layer 802 may be deposited on planarized top surface 800. In some embodiments, n+ polysilicon layer 802 is in an amorphous state as deposited. In other embodiments, n+ polysilicon layer 802 is in a polycrystalline state as deposited. CVD or another suitable process may be employed to deposit n+ polysilicon layer 802.
In an embodiment, n+ polysilicon layer 802 may be formed, for example, from about 100 to about 500 angstroms, of phosphorus or arsenic doped silicon having a doping concentration of about 1021 cm−3. Other layer thicknesses, doping types and/or doping concentrations may be used. N+ polysilicon layer 802 may be doped in situ, for example, by flowing a donor gas during deposition. Other doping methods may be used (e.g., implantation).
After deposition of n+ silicon layer 802, a doped p-type silicon layer 804 may be formed over n+ polysilicon layer 802. P-type silicon may be either deposited and doped by ion implantation or may be doped in situ during deposition to form a p-type silicon layer 804. For example, an intrinsic silicon layer may be deposited on n+ polysilicon layer 802, and a blanket p-type implant may be employed to implant boron a predetermined depth within the intrinsic silicon layer. Example implantable molecular ions include BF2, BF3, B and the like. In some embodiments, an implant dose of about 1-10×1013 ions/cm2 may be employed. Other implant species and/or doses may be used. Further, in some embodiments, a diffusion process may be employed. In an embodiment, the resultant p-type silicon layer 804 has a thickness of from about 800 to about 4000 angstroms, although other p-type silicon layer sizes may be used.
Following formation of p-type silicon layer 804, a heavily doped n+ polysilicon layer 806 is deposited on p-type silicon layer 804. In some embodiments, n+ polysilicon layer 806 is in an amorphous state as deposited. In other embodiments, n+ polysilicon layer 806 is in a polycrystalline state as deposited. CVD or another suitable process may be employed to deposit n+ polysilicon layer 806.
In an embodiment, n+ polysilicon layer 806 may be formed, for example, from about 100 to about 500 angstroms of phosphorus or arsenic doped silicon having a doping concentration of about 1021 cm−3. Other layer thicknesses, doping types and/or doping concentrations may be used. N+ polysilicon layer 806 may be doped in situ, for example, by flowing a donor gas during deposition. Other doping methods may be used (e.g., implantation). Persons of ordinary skill in the art will understand that silicon layers 802, 804 and 806 alternatively may be doped p+/n/p+, respectively, or may be doped with a single type of dopant to produce junctionless-FETs.
Following formation of n+ polysilicon layer 806, a first hard mask material layer 808 is deposited on n+ polysilicon layer 806, resulting in the structure shown in FIGS. 8B1-8B3. First hard mask material layer 808 may include any suitable hard mask material such as silicon nitride, amorphous carbon, or the like deposited by any suitable method (e.g., CVD, PVD, etc.). In at least one embodiment, first hard mask material layer 808 may comprise between about 300 angstroms and about 800 angstroms of silicon nitride. Other hard mask materials and/or thicknesses may be used.
First hard mask material layer 808 and silicon layers 802, 804 and 806 are patterned and etched to form rows 810 of silicon layers 802, 804 and 806 and first hard mask material layer 808, resulting in the structure shown in FIGS. 8C1-8C3. For example, first hard mask material layer 808 and silicon layers 802, 804 and 806 may be patterned and etched using conventional lithography techniques, with wet or dry etch processing.
First hard mask material layer 808 and silicon layers 802, 804 and 806 may be patterned and etched in a single pattern/etch procedure or using separate pattern/etch steps. Any suitable masking and etching process may be used to form vertical transistor pillars. For example, silicon layers may be patterned with about 1 to about 1.5 micron, more preferably about 1.2 to about 1.4 micron, of photoresist (“PR”) using standard photolithographic techniques. Thinner PR layers may be used with smaller critical dimensions and technology nodes. In some embodiments, an oxide hard mask may be used below the PR layer to improve pattern transfer and protect underlying layers during etching.
In some embodiments, after etching, rows 810 may be cleaned using a dilute hydrofluoric/sulfuric acid clean. Such cleaning may be performed in any suitable cleaning tool, such as a Raider tool, available from Semitool of Kalispell, Mont. Example post-etch cleaning may include using ultra-dilute sulfuric acid (e.g., about 1.5 1.8 wt %) for about 60 seconds and/or ultra-dilute HF acid (e.g., about 0.4-0.6 wt %) for 60 seconds. Megasonics may or may not be used. Other clean chemistries, times and/or techniques may be employed.
A sacrificial material layer 812 is deposited over substrate 502, filling voids between rows 810. For example, approximately 200 angstroms to about 500 angstroms of silicon dioxide 812 may be deposited and etched-back, resulting in the structure shown in FIGS. 8D1-8D3. Other sacrificial materials and thicknesses may be used.
A dielectric material 814 is deposited over substrate 502, filling voids between rows 810. For example, approximately 200 angstroms to about 500 angstroms of silicon dioxide 814 may be deposited and planarized using chemical mechanical polishing or an etchback process to form a planar surface 816, resulting in the structure shown in FIGS. 8E1-8E3. Other dielectric materials and thicknesses may be used.
Following formation of dielectric material 814, a second hard mask material layer 818 is deposited on planar surface 816, resulting in the structure shown in FIGS. 8F1-8F3. Second hard mask material layer 818 may include any suitable hard mask material such as silicon nitride, amorphous carbon, or the like deposited by any suitable method (e.g., CVD, PVD, etc.). In at least one embodiment, second hard mask material layer 818 may be between about 300 angstroms and about 800 angstroms of silicon nitride. Other hard mask materials and/or thicknesses may be used.
Second hard mask material layer 818, first hard mask material layer 808, and silicon layers 802, 804 and 806 are patterned and etched to form vertical transistor pillars, resulting in the structure shown in FIGS. 8G1-9G5. For example, second hard mask material layer 818, first hard mask material layer 808, and silicon layers 802, 804 and 806 may be patterned and etched using conventional lithography techniques, with wet or dry etch processing.
In an embodiment, second hard mask material layer 818, first hard mask material layer 808, and silicon layers 802, 804 and 806 are patterned and etched to form vertical transistor pillars disposed above global bit lines GBL1, GBL2, GBL3. The vertical transistor pillars will be used to form vertically-oriented bit line select transistors Q11-Q33.
Next, sacrificial material layer 812 is removed, for example by a wet or dry etch process, resulting in the structure shown in FIGS. 8H1-8H4.
A gate dielectric material layer 522 is deposited conformally over substrate 502, and forms on sidewalls of second hard mask material layer 818, first hard mask material layer 808, and silicon layers 802, 804 and 806, resulting in the structure shown in FIGS. 8I1-8I5. For example, between about 30 angstroms to about 100 angstroms of silicon dioxide may be deposited. Other dielectric materials such as silicon nitride, silicon oxynitride, low K dielectrics, etc., and/or other dielectric material layer thicknesses may be used.
A gate electrode material is deposited over gate dielectric material layer 522. For example, approximately 50 angstroms to about 500 angstroms of a highly-doped semiconductor, such as n+ polysilicon, p+ polysilicon, or other similar conductive material may be deposited. The as-deposited gate electrode material is subsequently etched back to form gate electrodes 612, resulting in the structure shown in FIGS. 8J1-8J5. Gate electrodes 612 are disposed on all four sides of the vertical stack of silicon layers 802, 804 and 806. Other conductive materials and/or thicknesses may be used for gate electrodes 612.
Each of gate electrodes 612 includes first and second bridge portions extending in a direction along a long axis (e.g., along the y-axis in FIGS. 7B and 7C) of gate electrodes 612. For example, referring to FIGS. 8J2 and 8J5, each of gate electrodes 612 includes first bridge portions 524a disposed near a top of gate electrodes 612, and second bridge portions 524b disposed near a bottom of gate electrodes 612. Without wanting to be bound by any particular theory, it is believed that having two bride portions, first bridge portions 524a and second bridge portions 524b may lower a resistance of gate electrodes 612.
In addition, without wanting to be bound by any particular theory, it is believed that first hard mask material layer 808 and second hard mask material layer 818 protects first bridge portions 524a and second bridge portions 524b during the etch-back process used to form gate electrodes 612.
Thus, the example process steps described above and illustrated in FIGS. 8A1-8J5 form vertically-oriented bit line select transistors Q11-Q33 that include surround gates formed by gate electrodes 612 disposed on all sides of the vertically-oriented bit line select transistors.
In particular, the surround gates of vertically-oriented bit line select transistors Q11, Q21 and Q31 are coupled to row select line SG1, the surround gates of vertically-oriented bit line select transistors Q12, Q22 and Q32 are coupled to row select line SG2, and the surround gates of vertically-oriented bit line select transistors Q13, Q23 and Q33 are coupled to row select line SG3.
In addition, each of gate electrodes 612 includes first bridge portions 524a disposed near a top of gate electrodes 612, and second bridge portions 524b disposed near a bottom of gate electrodes 612.
Following the example process illustrated in FIGS. 8A1-8J5, additional process steps may be used to form word lines WL10, WL43 arranged and vertical bit lines LBL11-LBL33 above vertically-oriented bit line select transistors Q11-Q33 and global bit lines GBL1-GBL3.
Thus, as described above, one embodiment of the disclosed technology includes a method that includes forming a transistor by forming a gate disposed in a first direction above a substrate. The gate includes a first bridge portion and a second bridge portion. The method includes forming the first bridge portion extending in the first direction and disposed near a top of the gate, and forming the second bridge portion extending in the first direction and disposed near a bottom of the gate.
One embodiment of the disclosed technology includes an apparatus that includes a monolithic three-dimensional memory array that has a vertically-oriented bit line select transistor that includes a gate disposed in a first direction above a substrate. The gate includes a first bridge portion that extends in the first direction and is disposed near a top of the gate, and a second bridge portion that extends in the first direction and is disposed near a bottom of the gate.
One embodiment of the disclosed technology includes a method that includes forming a vertically-oriented bit line select transistor including a stack of semiconductor material above a substrate, forming a gate disposed on all sides of the stack of semiconductor material, forming a first bridge portion of the gate extending in a first direction and disposed near a top of the gate, and forming a second bridge portion of the gate extending in the first direction and disposed near a bottom of the gate.
For purposes of this document, each process associated with the disclosed technology may be performed continuously and by one or more computing devices. Each step in a process may be performed by the same or different computing devices as those used in other steps, and each step need not necessarily be performed by a single computing device.
For purposes of this document, reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “another embodiment” may be used to described different embodiments and do not necessarily refer to the same embodiment.
For purposes of this document, a connection can be a direct connection or an indirect connection (e.g., via another part).
For purposes of this document, the term “set” of objects may refer to a “set” of one or more of the objects.
Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

## Claims

1. A method comprising:
forming a transistor by:
forming a gate disposed in a first direction above a substrate, the gate comprising a first bridge portion and a second bridge portion;
forming the first bridge portion extending in the first direction and disposed near a top of the gate; and
forming the second bridge portion extending in the first direction and disposed near a bottom of the gate.

2. The method of claim 1, wherein forming a transistor further comprises:
forming a vertical stack of semiconductor material;
forming the gate on all sides of the vertical stack of semiconductor material.

3. The method of claim 2, wherein forming a transistor further comprises forming a dielectric material between the gate and the sides of the vertical stack of semiconductor material.

4. The method of claim 2, wherein forming a vertical stack of semiconductor material comprises forming a first region having a first conductivity type, a second region having a second conductivity type and a third region having the first conductivity type.

5. The method of claim 1, wherein the transistor comprises a vertically-oriented field effect transistor.

6. The method of claim 1, wherein the gate comprises a surround gate of the transistor.

7. The method of claim 1, wherein the transistor comprises a first terminal, a second terminal and a third terminal, and the method further comprises coupling the first terminal to a vertical bit line of a memory array, coupling the second terminal to a global bit line of a memory array, and coupling the third terminal to the gate.

8. The method of claim 7, wherein memory array comprises a memory cell comprising a vertical strip of non-volatile memory material.

9. The method of claim 8, wherein the non-volatile memory material comprises any of a reversible resistance-switching material, a phase change material, and a charge trapping layer.

10. The method of claim 8, wherein the non-volatile memory material comprises one or more of HfO2, Al2O3, HfSiOx, HfSiOxNy, HfAlOx, Nb2O5, Ta2O5, ZrO2, Cr2O3, Fe2O3, Ni2O3, Co2O3, WO3, TiO2, SrZrO3, SrTiO3.

11. An apparatus comprising:
a monolithic three-dimensional memory array comprising a vertically-oriented bit line select transistor comprising a gate disposed in a first direction above a substrate, the gate comprising:
a first bridge portion extending in the first direction and disposed near a top of the gate; and
a second bridge portion extending in the first direction and disposed near a bottom of the gate.

12. The apparatus of claim 11, wherein the vertically-oriented bit line select transistor comprises a surround gate comprising the gate terminal.

13. The apparatus of claim 11, wherein the gate terminal comprises highly doped semiconductor material.

14. The apparatus of claim 11, wherein the vertically-oriented bit line select transistor comprises a first terminal coupled to a vertical bit line of the monolithic three-dimensional memory array, a second terminal coupled to a global bit line of the monolithic three-dimensional memory array, and a the third terminal coupled to a row select line of the monolithic three-dimensional memory array.

15. The apparatus of claim 11, wherein monolithic three-dimensional memory array comprises a vertical strip of non-volatile memory material.

16. The apparatus of claim 15, wherein the non-volatile memory material comprises any of a reversible resistance-switching material, a phase change material, and a charge trapping layer.

17. The apparatus of claim 15, wherein the non-volatile memory material comprises one or more of HfO2, Al2O3, HfSiOx, HfSiOxNy, HfAlOx, Nb2O5, Ta2O5, ZrO2, Cr2O3, Fe2O3, Ni2O3, Co2O3, WO3, TiO2, SrZrO3, SrTiO3.

18. A method comprising:
forming a vertically-oriented bit line select transistor comprising a stack of semiconductor material above a substrate;
forming a gate disposed on all sides of the stack of semiconductor material;
forming a first bridge portion of the gate extending in a first direction and disposed near a top of the gate; and
forming a second bridge portion of the gate extending in the first direction and disposed near a bottom of the gate.

19. The method of claim 18, wherein the gate, the first bridge portion and the second bridge portion comprise a highly doped semiconductor material.

20. The method of claim 18, wherein the vertically-oriented bit line select transistor comprises a first terminal, a second terminal and a third terminal, and the method further comprises coupling the first terminal to a vertical bit line of a monolithic three-dimensional memory array, coupling the second terminal to a global bit line of the monolithic three-dimensional memory array, and coupling the third terminal to a row select line of the monolithic three-dimensional memory array.