US 7595237 B2
A non-volatile memory cell includes an access and a storage transistor coupled in series. The memory cell is formed on a thin gate well tailored for transistors with thin gate dielectrics. The access transistor is a hybrid transistor which includes a gate with a thick gate dielectric layer formed on the thin gate well.
1. A method of fabricating an integrated circuit (IC) comprising:
providing a substrate with a cell region defined thereon, the cell region is prepared with a thin gate doped well tailored for transistors with thin gate dielectric layers; and
forming a non-volatile memory cell in the cell region, the non-volatile memory cell having an access transistor and a storage transistor, wherein the access transistor includes an access gate with an access gate dielectric comprising a thick gate dielectric layer on the thin gate doped well, wherein wells for transistors with thick gate dielectric layers have a lower dopant concentration than the thin gate doped well.
2. The method of
forming a storage gate in a storage portion of the cell region, the storage gate comprises a storage layer and a storage gate electrode over the storage layer; and
forming first and second storage diffusion regions adjacent to the storage gate.
3. The method of
forming the access gate comprising the access gate dielectric and an access gate electrode over the thick gate dielectric layer; and
forming first and second access diffusion regions adjacent to the access gate.
4. The method of
forming a storage layer in a storage portion of the cell region;
forming the access gate dielectric in an access portion of the cell region; and
forming a gate electrode layer over the storage layer and thick gate dielectric layer.
5. The method of
6. The method of
7. The method of
8. The method of
9. The method of
10. The method of
forming a storage layer in the cell region;
patterning the storage layer with a patterning mask to remove exposed portion of the storage layer from an access portion of the cell region;
selectively forming the access gate dielectric in the access portion of the cell region; and
depositing a gate electrode layer over the storage layer and access gate dielectric in the cell region.
11. The method of
12. The method of
13. The method of
14. The method of
patterning the layers over the cell region to form a storage gate and the access gate, wherein the storage gate comprises a gate electrode formed from the gate electrode layer over the storage layer and the access gate comprises an access gate electrode formed from the gate electrode layer over the access gate dielectric; and
forming first and second access diffusion regions adjacent to the access gate and first and second storage diffusion regions adjacent to the storage gate.
15. The method of
16. A method of forming a memory cell comprising:
providing a substrate with a cell region defined thereon, the cell region is prepared with a thin gate doped cell well tailored for thin gate dielectric transistors, the cell region comprises access and storage sub-regions;
forming a storage transistor in the storage sub-region, the storage transistor comprising a storage gate having a storage layer on the substrate and a storage gate electrode over the storage layer; and
forming an access transistor in the access sub-region on the thin gate doped cell well, the access transistor comprising a thick access gate dielectric on the substrate and a gate electrode on the thick access gate dielectric, wherein wells for transistors with thick gate dielectrics have a lower dopant concentration than the thin gate doped well.
17. A method of fabricating a semiconductor device comprising:
providing a substrate with an active region prepared with a doped cell well tailored for transistors with thin gate dielectric; and
forming a transistor in the active region on the thin gate doped well, wherein the transistor comprises a gate having a gate electrode over a thick gate dielectric, wherein wells for transistors with thick gate dielectrics have a lower dopant concentration than the thin gate doped well.
18. The method of
the thin gate doped well comprises n-type dopants and the transistor comprises a p-type transistor; or
the thin gate doped well comprises p-type dopants and the transistor comprises a n-type transistor.
19. The method of
20. The method of
the active region comprises a cell region of a memory cell; and
the memory cell comprises
an access transistor having an access gate electrode over a thick access gate dielectric, and
a storage transistor coupled to the access transistor, the storage transistor having a storage gate electrode over a storage gate dielectric.
21. The method of
the storage gate dielectric comprises an ONO storage stack; and
the thick access gate dielectric is about 30-80 Å thick.
The present invention relates generally to integrated circuits (ICs), and more particularly to non-volatile memory devices.
Non-volatile memory devices are able to retain stored data even when the power supply is interrupted. Non-volatile memory devices comprise flash devices which can be programmed using electrical signals. For the memory device to be byte-operational, a 2T cell structure consisting of a memory transistor and a select or access transistor in series can be provided. The memory transistor stores data programmed into the memory cell, while the access transistor selects the memory cell to be programmed or erased.
The memory transistor can be of various types including, for example, floating gate type, metal-nitride-oxide-silicon (MNOS) type, silicon-nitride-oxide-silicon (SNOS) type, metal-oxide-nitride-oxide-silicon (MONOS) type, and silicon-oxide-nitride-oxide-silicon (SONOS) type. The SONOS type memory transistor has a stacked gate structure comprising a nitride layer sandwiched between lower and upper oxide layers, and a polysilicon gate layer. The lower oxide layer is a tunnel oxide layer, the nitride layer is a memory or storage layer, and the upper oxide layer is a blocking layer for preventing the loss of stored charge. The charge-trapping ability of the SONOS stack structure allows lower programming and erase voltages to be used. The SONOS memory transistor also comprises source and drain regions formed on either side of the stack structure.
Programming is typically by hot carrier injection. Programming speed depends on the efficiency of hot carrier generation which, in turn, depends on the programming current. Programming current is affected by various factors including, for example, channel doping level and gate width of the memory transistor. Erase can be by Fowler-Nordheim (F-N) tunneling.
Generally, it is desirable to provide a non-volatile memory device having high programming speed and small cell size to allow for high density memories.
The present invention relates generally to ICs. In particular, the present invention relates to non-volatile memory devices. In one aspect of the invention, an IC is provided. The IC comprises a thin gate well with channel doping tailored for transistors with thin gate dielectric layers and a non-volatile memory cell formed on the thin gate well. The non-volatile memory cell comprises an access transistor and a storage transistor. The access transistor comprises first, second diffusion regions and an access gate which includes a thick gate dielectric layer. The storage transistor which is coupled in series to the access transistor comprises first, second diffusion regions and a storage gate. The storage gate includes a storage layer.
In another aspect of the invention, a method of fabricating an integrated circuit is disclosed. The method comprises providing a substrate with a cell region defined thereon. The cell region is prepared with a thin gate well tailored for transistors with thin gate layers. A non-volatile memory cell having an access transistor and a storage transistor is formed in the cell region. The access transistor includes an access gate with a thick gate dielectric layer.
These and other objects, along with advantages and features of the present invention herein disclosed, will become apparent through reference to the following description and the accompanying drawings. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations.
In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. Various embodiments of the present invention are described with reference to the following drawings, in which:
The present invention relates to non-volatile memory cells. More particularly, the present invention relates to a 2T flash type non-volatile memory device which increases programming speed while reducing cell size, enabling high density memories to be fabricated. The non-volatile memory cells can be incorporated into ICs and easily integrated into current CMOS processing technologies. The ICs can be any type of IC, for example non-volatile memories, signal processors, or system on chip devices. Other types of ICs are also useful. Such ICs are incorporated in, for example, microcontrollers, communication systems, consumer products such as cell phones and memory cards.
In accordance with one embodiment of the invention, the access transistor comprises a hybrid transistor. The hybrid transistor includes a thick gate oxide 128 with a doped well 115 to accommodate a transistor with a thin gate oxide. The doped well comprises second polarity type dopants. For example, the doped well comprises a n-well for p-type transistors. As for the storage transistor, it includes a composite charge storing layer 148. In one embodiment, the composite charge storage layer comprises an oxide-nitride-oxide (ONO) stack. Other types of storage layers, such as nanocrystal embedded in oxide and oxide-metal(high-K)-oxide stack are also useful.
As shown in
The cell region includes first and second transistors 320 and 340, with the first transistor serving as an access transistor and the second transistor serving as a storage transistor. The transistors, in one embodiment, comprise transistors of a first polarity type. In one embodiment, the transistors comprise p-type transistors. N-type transistors are also useful. Each transistor includes first and second diffusion regions (322, 323, 342, 343) adjacent to a gate stack (325, 345). The diffusion regions are formed by first polarity type dopants. Preferably, the transistors share a common second diffusion region 323/343. Contacts, such as silicide contacts, can be provided for the diffusion regions. A doped well 315 is provided beneath the substrate in the cell region. The doped well comprises a second polarity type doped well.
Disposed on the substrate in the cell region is the gate stack of the access transistor. The gate stack includes a gate layer 327 over a gate dielectric layer 328. The gate layer comprises, for example, polysilicon. As for the gate dielectric layer, it can comprise various types of dielectric materials. Preferably, the gate dielectric comprises thermal oxide. In one embodiment, the gate dielectric layer comprises a thick gate dielectric layer, with a physical thickness typically about 30-80 Å. Other thicknesses may also be useful. In accordance with one embodiment of the invention, the doped well on which the access transistor is formed comprises a doped well tailored for a transistor with a thin gate dielectric layer (thin gate well), providing a hybrid access transistor. The thin gate doped well, for example, comprises a dopant concentration of about E17-E19 cm−3, and a depth of about 0.7-1.4 μm. Other concentrations and depths are also useful.
On the surface of the substrate in the cell region over the doped well is also disposed the gate stack of the storage transistor. Preferably both the access and storage transistors are formed in the same well. Forming the transistors in a common well region enables the memory cell to be reduced in size. The gate stack comprises a gate layer 347 over a storage dielectric layer 348. Typically, the gate layer comprises polysilicon. Other types of materials can also be used to form the gate layer.
In one embodiment, the storage layer comprises an oxide-nitride-oxide (ONO) trilayer, forming a SONOS gate stack structure. The ONO trilayer includes a lower (tunneling) oxide layer 331, an upper (blocking) oxide layer 333, and a nitride layer 332 sandwiched therebetween. Other dielectric materials capable of storing electrical charges may also be used. Additionally, the storage layer may comprise one, two or more layers. For example, a bilayer storage layer comprising a nitride layer and a tunneling oxide layer can be used. To facilitate tunneling and trapping of electrical charges for efficient programming and erasing of the non-volatile memory, the lower oxide layer is preferably about 24-45 Å physically thick, the nitride layer is preferably about 40-80 Å thick, and the upper oxide layer is preferably about 40-80 Å thick. More preferably, the lower oxide layer, nitride layer, and upper oxide layer are, respectively, about 35 Å, 45 Å and 45 Å thick. Other types of flash memories can also be used.
Although not shown, an interlevel dielectric layer is provided over the memory cell. Contacts are provided, coupling the diffusion regions and gate electrodes to source lines, bit lines, access gate lines and control gate lines.
As described, a non-volatile memory cell includes a hybrid access transistor with a thick gate dielectric layer formed on a thin gate well. Thin gate wells have higher channel doping compared to thick gate wells (wells tailored for transistors with thick gate dielectric layers), resulting in higher carrier generation efficiency. Furthermore, the thick gate dielectric layer of the hybrid access transistor allows higher bias voltages to be used. Higher bias voltages result in increased programming currents, which improve programming speed.
Preferably, the gate length of the storage transistor is reduced to increase tunneling current for higher programming speed. Furthermore, the gate length of the access transistor is reduced to be the thin gate channel length, resulting in smaller memory cell sizes. The gates, however, must be of sufficient length to prevent punch-through. Since immunity to punch-through increases with increased channel doping concentration, the use of thin gate wells allows transistors to have smaller gate lengths than thick gate wells. The gate lengths of the transistors, in a preferred embodiment, are selected to be at the minimum critical dimension (CD) for a specific process. More preferably, the minimum CD of each type of transistor is used. For example, for 0.18 μm technology, the gate length of the access transistor is about 0.18 μm while that of the storage transistor is about 0.17 μm.
The memory cell can operate in various modes, such as programming, reading and erase modes. The erase mode is initiated by applying appropriate bias conditions. In one embodiment, the erase mode is initiated by applying about 3.3 V to the gate of the access transistor, about 6 V to the thin gate n-well, and about −6.25 V to the gate of the storage transistor. Under such bias condition, data erasing is performed using a F-N tunneling current that flows through the tunneling oxide, drawing electrons from the ONO stack of the non-volatile memory transistor to the channel region. The bias voltage (Vgb) during erase, which is equal to the difference between the access transistor operating voltage and the n-well voltage, is about −2.7 V. Operation in this low Vgb region avoids the problems of oxide breakdown, trap-up or transistor degradation associated with high electric field stressing.
The use of a thick gate oxide in the access transistor allows the access transistor to operate at 3.3 V. Conversely, if a thin gate oxide were used, the access transistor can only operate at a maximum of 1.8 V if gate oxide rupture is to be avoided. In such case, Vgb is about −4.2 V in the erase mode since 6 V is applied to the thin gate n-well. Such a high bias voltage is liable to cause gate breakdown and reliability problems. Accordingly, the use of a hybrid access transistor is thus able to provide favorable bias conditions.
The substrate is prepared with a doped well 515 in the cell region. The doped well comprises dopants of the second polarity type. The second polarity type, for example, comprises n-type, forming a n-well. Alternatively, the second polarity type comprises p-type. In accordance with one embodiment of the invention, the doped well comprises a thin gate well having a channel doping concentration of about E17-E19 cm−3. Conventional ion implantation techniques, such as implantation with a mask can be used to form the wells. Generally, the substrate includes first and second type wells in the logic region to form, for example, CMOS devices. Separate ion implantation processes are used for different types of wells. The doped well in the cell region can be formed along with the same type of wells in the logic regions. Threshold voltage (VT) implants are performed after well formation.
Isolation regions (not shown), such as STIs, are provided to isolate the cell region from other device regions. The STIs can be formed before or after the well is formed. In one embodiment, the STIs are formed before the wells are formed.
In one embodiment, the array region is masked off for processing of the logic region. For example, portions of the storage layer in the logic region are removed, if applicable, and conventional dual gate processes are performed therein. For example, a thin gate dielectric layer is formed in the logic region. Since the array region is covered by mask layer during these processes, concerns about overlay or gate integrity issues are eliminated. Dual gate processes are described in, for example, U.S. Pat. No. 7,029,976, which is incorporated herein by reference.
The process continues by forming interconnections to the diffusion regions and gates of the transistors. For example, the diffusion regions and gates are appropriately coupled to source lines, bit lines, access lines and control lines. Additionally, the thin gate well is coupled to a bias voltage. Appropriate interconnections are also formed in the logic region. Additional processes are performed to complete the IC. These processes include, for example, final passivation, dicing, and packaging.
As described, the memory cell of the present invention can be easily integrated into conventional CMOS processes. By forming and patterning the thick gate dielectric layer in the logic and cell regions concurrently, the additional gate steps are reduced or minimized. No additional implant processes are required to form the hybrid access transistors. By providing a memory cell design which facilitates simple circuit routing along with minimum CD gate lengths, cell size is reduced and density is increased. Furthermore, masking off the array region during dual gate processes eliminates overlay or gate integrity issues in the array region. In addition, the thick gate oxide of the access transistor allows for appropriate bias conditions to increase programming speed reliably. Therefore, the present invention provides a simple, reliable, high performing and low cost solution for integrating non-volatile memories in ICs.
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments, therefore, are to be considered in all respects illustrative rather than limiting the invention described herein. Scope of the invention is thus indicated by the appended claims, rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.