# Patent

## Title
Integration of non-volatile charge trap memory devices and logic CMOS devices

## Abstract

A semiconductor structure and method to form the same. The semiconductor structure includes a substrate having a non-volatile charge trap memory device disposed on a first region and a logic device disposed on a second region. A charge trap dielectric stack may be formed subsequent to forming wells and channels of the logic device. HF pre-cleans and SC1 cleans may be avoided to improve the quality of a blocking layer of the non-volatile charge trap memory device. The blocking layer may be thermally reoxidized or nitridized during a thermal oxidation or nitridation of a logic MOS gate insulator layer to densify the blocking layer. A multi-layered liner may be utilized to first offset a source and drain implant in a high voltage logic device and also block silicidation of the nonvolatile charge trap memory device.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No. 12/125,864, filed May 22, 2008, the entire contents of which are hereby incorporated by reference herein. This application claims the benefit of U.S. Provisional Application No. 60/940,148, filed May 25, 2007, the entire contents of which are hereby incorporated by reference herein. This application also claims the benefit of U.S. Provisional Application No. 60/940,137, filed May 25, 2007, the entire contents of which are hereby incorporated by reference herein.


TECHNICAL FIELD
The invention is in the field of semiconductor devices, more specifically pertaining to non-volatile charge trap memory devices integrated with logic CMOS devices.
BACKGROUND
Feature scaling in integrated circuits is an enabler of more capable electronic devices. Scaling to smaller features increases densities of functional units in a given form factor as well as increasing device processing speeds. Device scaling, however, is not without issue. For example, optimizing the performance of smaller devices becomes increasingly difficult. This is particularly true for the scaling of non-volatile charge trap memory devices, in which data retention and sensing becomes increasingly difficult as the devices are scaled.
In addition to device scaling, system-on-a-chip type architecture also increases electronic device functionality. Such architecture may incorporate, for example, a memory device on the same substrate as a logic device to reduce the cost of fabrication as well as increase communication bandwidth between the memory and logic devices.
The integration of these dissimilar devices in a system-on-a-chip architecture is problematic because the fabrication process for the logic MOS device may hamper the fabrication process of the memory device and visa versa. Such a dilemma may occur, for example, when integrating the logic MOS gate oxide process module with the fabrication of a dielectric stack for a memory device. Also, channel and well implant processing for the logic devices may also be detrimental to the memory device dielectric stack while formation of the latter may be problematic for the former. As still another example, silicided contacts, which are advantageous for a logic transistor, may adversely affect a nonvolatile charge trap memory device.
Also, operation of a non-volatile memory device may require application of relatively high voltages (HV), typically of at least 10 V. However, the conventional processes employed in fabrication of a scaled logic device are typically optimized for device operation at 5 V or less. Such low voltage devices may lack a sufficiently high breakdown voltage to interface directly with a memory device.



BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention are illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which:
FIG. 1 illustrates a flow diagram depicting sequences of particular modules employed in the fabrication process of a non-volatile charge trap memory device integrated with a logic MOS fabrication process, in accordance with particular embodiments of the present invention;
FIGS. 2A and 2B illustrate flow diagrams depicting sequences of particular operations in the integration of logic MOS gate fabrication with a non-volatile charge-trapping dielectric stack for implementing certain modules illustrated in FIG. 1, in accordance with particular embodiments of the present invention; and
FIG. 3A illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a SONOS channel implant is performed while a screening oxide is over the MOS and HV MOS regions of a substrate, in accordance with an embodiment of the present invention;
FIG. 3B illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a SONOS charge trapping dielectric stack is formed and the MOS and HV MOS regions are cleaned in preparation for forming a first gate insulator layer, in accordance with an embodiment of the present invention;
FIG. 3C illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a first gate insulator layer is formed over the MOS and HV MOS regions, in accordance with an embodiment of the present invention;
FIG. 3D illustrates a cross-sectional view representing operations wherein SONOS and HV MOS device regions are masked while the first gate insulator layer in the MOS region is opened in a third region of the substrate to form a second gate insulator layer, in accordance with an embodiment of the present invention;
FIG. 3E illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a second gate insulator layer is formed in the MOS region, in accordance with an embodiment of the present invention;
FIG. 3F illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which the SONOS oxide blocking layer, the HV MOS gate insulator layer and the MOS gate insulator layer are nitrided, in accordance with an embodiment of the present invention;
FIG. 3G illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a gate layer is deposited, in accordance with an embodiment of the present invention;
FIG. 3H illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a gate electrode is formed in accordance with an embodiment of the present invention;
FIG. 3I illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a sidewall spacer is formed in accordance with an embodiment of the present invention; and
FIG. 3J illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which charge trap dielectric and gate dielectric is removed adjacent to sidewall spacers to complete definition of gate stacks, in accordance with an embodiment of the present invention.
FIG. 4A illustrates a cross-sectional view representing operations in the formation of a semiconductor structure having a SONOS gate stack with adjacent sidewall spacers as wells as HV MOS and MOS device gate stacks with adjacent sidewall spacers on a single substrate, in accordance with an embodiment of the present invention;
FIG. 4B illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a multi-layered liner is deposited over the SONOS and logic devices, in accordance with an embodiment of the present invention;
FIG. 4C illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which the top layer of the multi-layered liner is etched to form a disposable spacer, in accordance with an embodiment of the present invention;
FIG. 4D illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a HV MOS device receives a source and drain implant while the SONOS and MOS devices are masked, in accordance with an embodiment of the present invention;
FIG. 4E illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which the disposable spacer is removed from the SONOS and logic devices, in accordance with an embodiment of the present invention;
FIG. 4F illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which the bottom layer of the multi-layered liner is removed from the MOS device but retained over the SONOS and HV MOS devices, in accordance with an embodiment of the present invention;
FIG. 4G illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a silicide is formed on the MOS device but blocked by the bottom layer of the multi-layered liner over the SONOS and MOS devices, in accordance with an embodiment of the present invention;
FIG. 5 illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which an interlayer dielectric (ILD) layer is formed on the sidewalls of the sidewall spacers adjacent to the MOS SONOS and HV MOS gate stacks, in accordance with an embodiment of the present invention;
FIG. 6A illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a stress inducing ILD layer is formed on the sidewall spacers adjacent to the MOS gate stack and formed on a bottom layer of the multi-layered liner over the SONOS and HV MOS gate stacks, in accordance with an embodiment of the present invention; and
FIG. 6B illustrates a cross-sectional view representing operations in the formation of a semiconductor structure in which a low-stress ILD layer is formed on a bottom layer of the multi-layered liner covering the sidewall spacers adjacent to the SONOS and HV MOS gate stacks and formed on a stress inducing ILD layer over the MOS device, in accordance with an embodiment of the present invention.



DETAILED DESCRIPTION
Embodiments of a non-volatile charge trap memory device integrated with logic devices are described herein with reference to figures. However, particular embodiments may be practiced without one or more of these specific details, or in combination with other known methods, materials, and apparatuses. In the following description, numerous specific details are set forth, such as specific materials, dimensions and processes parameters etc. to provide a thorough understanding of the present invention. In other instances, well-known semiconductor design and fabrication techniques have not been described in particular detail to avoid unnecessarily obscuring the present invention. Reference throughout this specification to “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.
The terms “over,” “under,” “between,” and “on” as used herein refer to a relative position of one layer with respect to other layers. As such, for example, one layer deposited or disposed over or under another layer may be directly in contact with the other layer or may have one or more intervening layers. Moreover, one layer deposited or disposed between layers may be directly in contact with the layers or may have one or more intervening layers. In contrast, a first layer “on” a second layer is in contact with that second layer. Additionally, the relative position of one layer with respect to other layers is provided assuming operations deposit, modify and remove films relative to a starting substrate without consideration of the absolute orientation of the substrate.
In an embodiment, a nonvolatile charge trap dielectric stack, such as a SONOS stack, is formed in a first region of a substrate after at least some of the well and channel implants of logic MOS devices are formed in a second region of the substrate.
In another embodiment, a nonvolatile charge trap dielectric stack is formed prior to any logic MOS gate oxidation processing. In one such embodiment, a SONOS stack is removed from the second region of the substrate, and a thermal oxidation forms a first gate insulator layer over the second region of the semiconductor substrate and thermally reoxidizes a blocking layer of the SONOS stack. In a further embodiment, a nitridation process nitridizes the first gate insulator layer and the blocking layer simultaneously.
In another embodiment, a nonvolatile charge trap memory device without silicide contacts is integrated with a logic device having silicide contacts. Such an embodiment may advantageously improve the reliability of the nonvolatile charge trap memory device by reducing silicide-related stress in the memory device.
In a further embodiment, at least one of the logic devices has a longer lightly doped source and drain (i.e. offset source and drain) than at least another one of the logic devices to allow for HV operation (e.g. breakdown voltage greater than 10 V). In one such embodiment, wherein the logic devices include a HV PMOS device and a n-type MOS (NMOS) device, the NMOS device has a smaller source and drain offset than does the HV PMOS device. In another such embodiment, wherein the logic devices include a HV PMOS device and a PMOS device, the PMOS device has a smaller source and drain offset than does the HV PMOS. In a particular embodiment, the lightly doped source and drain of the HV MOS device is a length greater than the thickness of a sidewall spacer adjacent to a sidewall of a gate stack of the MOS device.
In another embodiment, a multi-layered liner is employed to offset the HV MOS source and drain and also protect the nonvolatile charge trap memory device from silicidation. In one such embodiment, wherein the multi-layered liner includes at least a top and bottom layer, a top layer is formed into a disposable spacer to offset the HV MOS source and drain and the bottom layer is used to mask the nonvolatile charge trap memory device during a silicidation of one or more of the logic devices. In another embodiment, the bottom layer is additionally used to mask the HV MOS device during silicidation of one or more of the logic devices. In a particular embodiment, the disposable spacer is removed selectively to the bottom layer of the multi-layered liner after the HV MOS source and drain are implanted. In a further embodiment, the bottom layer of the multi-layered liner is retained over the nonvolatile charge trap memory device as an ILD layer, covered with another ILD layer and then etched through during contact formation. In another embodiment, the bottom layer of the multi-layered liner is retained over the nonvolatile charge trap memory device and the HV MOS device as an ILD layer. In one such embodiment, the stress in the bottom layer of the multi-layered liner is of opposite sign than that of a stress inducing ILD layer deposited over the bottom liner layer. In one particular embodiment, the bottom layer of the multi-layered liner induces compressive stress on the underlying device while the stress inducing ILD layer induces tensile stress on the underlying device.
FIG. 1 illustrates a flow diagram depicting sequences of particular modules employed in the fabrication process 100 of a non-volatile charge trap memory device integrated with a logic MOS device, in accordance with particular embodiments of the present invention. The methods begin with formation of isolation regions at module 101. Isolation regions may be formed by any conventional technique, such as, but no limited to shallow trench isolation (STI) or local oxidation of silicon (LOCOS).
After the isolation regions are formed at module 101 the process flow may either proceed with well and/or channel implants at module 105 or delay the formation of the wells and/or channels until after formation of the non-volatile charge trapping dielectric stack and/or gate layer deposition.
In an advantageous embodiment, a non-volatile charge trapping dielectric stack is formed on a first region of a substrate at module 110 after at least some of the well and channel implants for the logic MOS transistors are formed at module 105. It has been found that approximately 0.5 nm of silicon dioxide may be removed during a conventional post-implant resist strip process. The amount removed is greater if the silicon dioxide is a deposited oxide rather than a thermally grown oxide or if the silicon dioxide received an implant (e.g. 1.0 nm of silicon dioxide removed/strip process). Because there is typically a number of well and channel implants in a CMOS process flow, between 1.5 nm and 2.5 nm of silicon dioxide may be removed prior to performing the RCA cleans in preparation for a logic MOS gate dielectric. Similarly, the well and channel implant strips may also etch a non-volatile charge trapping dielectric stack (which may include silicon dioxide). While the nominal etch rate of the implant strip processes is quite small, it has also been found to form pin holes, or localized defects in the non-volatile charge trapping dielectric stack which may reduce the charge retention of a SONOS-type memory device. Thus, inserting the module forming the non-volatile charge trapping dielectric stack after the well and channel implant modules of a logic CMOS flow results in the least disruption to the non-volatile charge trapping dielectric stack.
To include a logic MOS transistor on the same substrate as the non-volatile charge trap device, a gate insulator layer is formed on the second region of the substrate at module 120. As will be discussed in more detail subsequently, this sequence of forming the non-volatile charge trapping dielectric layer prior to forming the MOS gate insulator layer advantageously utilizes the subsequent thermal treatments forming the MOS gate insulator layer to improve the quality of the non-volatile charge trapping dielectric stack, particularly a blocking layer. Logic MOS transistor degradation from thermal processing associated with formation of the non-volatile charge trapping dielectric layers is also avoided by forming the non-volatile charge trapping dielectric stack prior to forming the logic MOS gate insulator layer.
Following module 120, a gate layer is deposited over both the MOS gate insulator layer and over the non-volatile charge trapping dielectric stack at module 130. Next, if the well and/or channel implants were not performed at module 105, the well and/or channel implants may be performed at module 140, after module 130. In such an embodiment, the well and channel implants may advantageously dope the gate layer formed at module 130 in addition to forming the wells and/or channels. With the well and channels in place (either as a result of module 105 or module 140), the gate layer may be then be patterned into gate electrodes at module 150. Gate electrode patterning may occur simultaneously for both a non-volatile charge trap memory device in the first region of the substrate and a MOS device in the second region of the substrate.
With the gate stacks of the two devices substantially complete at module 150, tip and/or HALO implants may be formed for all devices at module 155 and sidewall spacers formed for all devices at module 160. Source and drain implants may then be formed for all devices at operation 165. As described elsewhere herein, a multi-layered liner and disposable spacer process may be performed at these operations to provide a high voltage CMOS transistor. Then at operation 170, a silicide process may be performed to substantially complete the front end device fabrication. As further described herein, a multi-layered liner may be utilized to provide silicidation of logic CMOS without silicidation of the non-volatile charge trap memory device (i.e. selective silicidation). Backend metallization, as is conventional in the art, may then be performed to fabricate an integrated semiconductor structure comprising a non-volatile charge trap memory device and a MOS device on a single substrate.
FIG. 2A illustrates a flow diagram depicting fabrication process 200 including particular modules integrating formation of a charge-trapping dielectric stack with logic MOS gate insulator formation, in accordance with particular embodiments of the present invention. Thus, FIG. 2A depicts particular process modules employed in certain implementations of the modules 105, 110, 120 and 130 of FIG. 1. FIGS. 3A through 3J further illustrate a cross-section of a SONOS memory device, a high voltage MOS device and a low voltage MOS device as the modules in the process flow of FIG. 2A are implemented.
FIG. 2B illustrates a flow diagram depicting fabrication process 201 including particular modules integrating formation of a charge-trapping dielectric stack with a high voltage MOS transistor and with selective contact silicidation, in accordance with particular embodiments of the present invention. Thus, FIG. 2B depicts particular modules employed in certain implementations of the modules 155, 160, 165 and 170 of FIG. 1. FIGS. 4A through 6B further illustrate a cross-section of the non-volatile charge trap memory device, a high voltage MOS device and a low voltage MOS device as the modules in the process flow of FIG. 2B are implemented.
Referring to FIG. 2A, process 200 begins with STI formed in a substrate. The substrate may be a bulk substrate comprised of a single crystal of a material which may include, but is not limited to, silicon, germanium, silicon/germanium or a III-V compound semiconductor material. In another embodiment, the substrate is comprised of a bulk layer with a top epitaxial layer. In a specific embodiment, the bulk layer is comprised of a single crystal of a material which may include, but is not limited to, silicon, germanium, silicon/germanium, a III-V compound semiconductor material and quartz, while the top epitaxial layer is comprised of a single crystal layer which may include, but is not limited to, silicon, germanium, silicon/germanium and a III-V compound semiconductor material. In another embodiment, the substrate is comprised of a top epitaxial layer on a middle insulator layer which is above a lower bulk layer. The top epitaxial layer is comprised of a single crystal layer which may include, but is not limited to, silicon (i.e. to form a silicon-on-insulator (SOI) semiconductor substrate), germanium, silicon/germanium and a III-V compound semiconductor material. The insulator layer is comprised of a material which may include, but is not limited to, silicon dioxide, silicon nitride and silicon oxy-nitride. The lower bulk layer is comprised of a single crystal which may include, but is not limited to, silicon, germanium, silicon/germanium, a III-V compound semiconductor material and quartz.
At module 205, a first well implant, such an n-well implant is performed. Module 205 will typically include forming a patterned photomask on a screening sacrificial dielectric layer, such as a silicon dioxide layer. The n-well implant is then performed in a region of the substrate, such as the region for MOS transistor 370. In certain embodiments, the n-well implant includes implanting a phosphorus species at concentrations and energies conventional for MOS devices. A single n-well implant may be performed for PMOS transistors, PMOS HV transistors and p-type SONOS devices.
At module 206, a dry and/or wet strip is performed to remove the well implant photomask. Conventional plasma strips, such as oxygen, forming gas, and the like may be employed. Similarly, conventional wet strips, such as piranha clean and ozone clean may be used. Because the charge trapping dielectric stack of the non-volatile memory device has not yet been formed, the silicon dioxide etch rate of the strip module 206 is of little concern.
A module 207, a p-well implant is performed. Here too, conventional implant species, such as boron, may be employed at typical doses and energies. The p-well implant may be, but is not necessarily, a patterned implant such as the n-well implant of module 205. If patterned, any of those strip processes of module 206 may be repeated. In one embodiment, the p-well implant is performed in another area of the substrate, adjacent to an n-well region in preparation for an NMOS transistor. In alternative embodiments, the p-well implant is an unmasked implant.
Any number of channel implants may also be performed at module 207 to adjust threshold voltages for specific device applications. For example, an n-channel implant may be performed in a region of the substrate where a NMOS transistor channel will be located, thereby setting a threshold voltage. The n-channel implant may be of any conventional species (e.g. BF2), dose and energy for a particular device type. A channel implant for a non-volatile charge trap memory device may also be performed in a first region of the substrate 302, such as the region for SONOS device 300 of FIG. 3A. Similarly, a channel implant for a high voltage MOS transistor may be performed in the substrate region of HV MOS transistor 350. A p-channel implant may likewise be performed, for example in the substrate region of MOS transistor 370.
In the exemplary embodiment depicted in FIG. 3A, in preparation for a SONOS channel implant, a window 305 defined by photoresist 307 is formed in the sacrificial dielectric layer 303. The window 305 may be of sub-micron dimension, for example, approximately 0.2 um in length and width. In one such an embodiment, an oxygen plasma clean is performed to descum photoresist residue from the corners of window 305. A sacrificial silicon oxide layer, which in one exemplary implementation is between 10 and 30 nm thick, may then be removed with a buffered oxide etchant (BOE) containing a surfactant, again to ensure window 305 is opened completely.
The isotropic etch of the screening sacrificial dielectric layer 303 can be expected to undercut the photoresist 307 by an amount D1. The undercut amount D1 is important when window 305 is proximate to a logic device, such as in the region for HV MOS transistor 350 because logic device implants performed through the screening sacrificial dielectric layer 303 may have a different implant profile within the undercut region. Therefore, certain embodiments downsize the dimensions of window 305. For example a 0.2 um drawn size may be downsized to 0.18 um to compensate for an undercut of 0.01 um on a side. In further embodiments, because the undercut of window 305 may become very close to an adjacent logic device, critical layer lithography tools are employed to reduce misregistration tolerances.
After formation of the window 305, the channel implant may be performed and the photoresist 307 may be stripped. With the well and channel implants formed, an anneal may be performed to complete module 207. In one such embodiment, a rapid thermal anneal is performed after implanting both the n-well and p-well. The rapid thermal anneal may be any known in the art to be suitable for MOS transistor applications.
With the well and channel implants completed, the non-volatile charge trapping dielectric stack is formed at module 210. Referring back to FIG. 2A, a non-volatile charge trapping dielectric stack, such as an ONO charge trapping dielectric stack is then formed and patterned to remain only in memory cell areas at module 210. In one embodiment depicted in FIG. 3B, a ONO charge trapping dielectric stack 306 is comprised of a tunneling layer 304A, a charge trapping layer 304B and a blocking layer 304C. The tunneling layer 304A may be any material and have any thickness allowing charge carriers to tunnel into the charge-trapping layer under a high gate bias condition while maintaining a suitable barrier to leakage under conditions of low gate bias. In certain embodiments, tunneling layer 304A is a commonly known dielectric layer, such as silicon dioxide (SiO2), a silicon oxynitride (SiOxNy(Hz)), a silicon dioxide that is subsequently nitridized, or a stack dielectric made of silicon dioxide and silicon nitride (Si3N4) or silicon oxynitride, having a physical thickness of between about 1.5 nm and 3.0 nm. In another embodiment, tunneling layer 304A is comprised of a dielectric layer having a dielectric constant greater than that of silicon nitride which may include, but is not limited to, hafnium oxide, zirconium oxide, hafnium silicate, hafnium oxy-nitride, hafnium zirconium oxide and lanthanum oxide.
The charge trapping layer 304B of the SONOS device 300 may further include any commonly known charge trapping material and have any thickness suitable to store charge and, modulate the threshold voltage of the devices. In certain embodiments charge trapping layer 304B is silicon nitride, silicon-rich silicon nitride, or silicon oxynitride. In one particular embodiment, the trapping layer 304B has a non-uniform stoichiometry across the thickness of trapping layer. For example, the charge trapping layer 304B may further include at least two silicon oxynitride layers having differing compositions of silicon, oxygen and nitrogen. In one particular embodiment, a bottom oxynitride within charge trapping layer 304B has a first composition with a high silicon concentration, a high oxygen concentration and a low nitrogen concentration to provide an oxygen-rich oxynitride.
As used herein, the terms “oxygen-rich” and “silicon-rich” are relative to a stoichiometric silicon nitride, or “nitride,” commonly employed in the art having a composition of (Si3N4) and with a refractive index (RI) of approximately 2.0. Thus, “oxygen-rich” silicon oxynitride entails a shift from stoichiometric silicon nitride toward a higher wt % of silicon and oxygen (i.e. reduction of nitrogen). An oxygen-rich silicon oxynitride film is therefore more like silicon dioxide and the RI is reduced toward the 1.45 RI of pure silicon dioxide. Similarly, films described herein as “silicon-rich” entail a shift from stoichiometric silicon nitride toward a higher wt % of silicon with less oxygen than an “oxygen-rich” film. A silicon-rich silicon oxynitride film is therefore more like silicon and the RI is increased toward the 3.5 RI of pure silicon.
The bottom silicon oxynitride may have a physical thickness between 2.5 nm and 4.0 nm corresponding to an EOT of between 1.5 nm and 5.0 nm. The charge trapping layer 304B may further include a top silicon oxynitride with a high silicon concentration, a high nitrogen concentration and a low oxygen concentration to produce a silicon-rich silicon oxynitride. This second silicon oxynitride may have a physical thickness of 4.0 to 6.0 nm for a charge trapping layer 304B with a net physical thickness of 9 to 11 nm. The oxygen-rich stoichiometry of the first silicon oxynitride, being more like silicon dioxide relative to silicon nitride, provides a good quality interface with tunneling layer 304A. In one such embodiment, the composition of the oxygen-rich oxynitride results in an RI in the range of 1.7 and 1.9 and preferably about 1.8. In a further embodiment, the composition of the silicon-rich oxynitride results in an RI in the range of 1.8 and 2.0 and preferably about 1.9.
In one embodiment, multiple silicon nitride or silicon oxynitride charge trapping layers are formed in a low pressure CVD process using a silicon source, such as silane (SiH4), dichlorosilane (SiH2Cl2), tetrachlorosilane (SiCl4) or Bis-TertiaryButylAmino Silane (BTBAS), a nitrogen source, such as N2, NH3, N2O or nitrogen trioxide (NO3), and an oxygen-containing gas, such as O2 or N2O.
In one exemplary implementation, a silicon oxynitride charge trapping layer can be deposited over a tunneling layer by placing the substrate in a deposition chamber and the flow rate of ammonia (NH3) gas and nitrous oxide (N2O) as mixed with a silicon precursor, such as dichlorosilane (SiH2Cl2), to provide the desired gas ratios to form first an oxygen-rich oxynitride film and then a silicon-rich oxynitride film. Just as the terms “oxygen-rich” and “silicon-rich” are relative to a stoichiometric Si3N4 film, formation of these films may also be characterized based on the 3:1 volumetric flow rate ratio, SiH2Cl2:NH3, commonly employed to produce a stoichiometric (Si3N4) with a CVD method. The oxygen-rich oxynitride film is therefore formed with a relatively higher volumetric flow rate of oxidizer (e.g. N2O) than used for the silicon-rich oxynitride film while the both the oxygen-rich and silicon-rich oxynitride films are formed with a relatively higher volumetric flow rate of silicon precursor (e.g. SiH2Cl2).
In a specific batch process embodiment, an oxygen-rich oxynitride film is formed by introducing a process gas mixture including N2O, NH3 and SiH2Cl2, while maintaining the chamber at a pressure approximately in the range of 5-500 mTorr, and maintaining substrate 400 at a temperature approximately in the range of 700-850° C., for a period approximately in the range of 2.5-20 minutes. In an exemplary embodiment, the process gas mixture includes N2O and NH3 at a high volumetric flow rate ratio of about 1:1 to about 3:1 N2O:NH3 while the SiH2Cl2 to NH3 is also at a high volumetric flow rate ratio from about 3.5:1 to 8:1 SiH2Cl2:NH3. In a preferred embodiment, the N2O:NH3 ratio is about 2:1 while the SiH2Cl2:NH3 is at a ratio of about 6:1. In certain embodiments, the gases are introduced at a flow rate approximately in the range of 5-200 standard cubic centimeters per minute (sccm).
In a further embodiment, a silicon-rich oxynitride film is then formed by introducing a process gas mixture including N2O, NH3 and SiH2Cl2, while maintaining the chamber at a pressure approximately in the range of 5-500 mTorr, and maintaining substrate 400 at a temperature approximately in the range of 700-850° C., for a period approximately in the range of 2.5-20 minutes in a batch furnace. The process gas mixture includes N2O and NH3 at a volumetric flow rate ratio from about 1:8 to about 1:4 (N2O:NH3) with SiH2Cl2 and NH3 at a volumetric flow rate ratio from about 3.5:1 to 5:1 (SiH2Cl2:NH3). In a preferred embodiment, the N2O and NH3 are provided at a volumetric flow rate ratio of about 1:5 (N2O:NH3) while the SiH2Cl2 and NH3 are at a volumetric flow rate ratio of about 4:1 (SiH2Cl2:NH3). In certain embodiments, the gases are introduced at a flow rate approximately in the range of 5 to 200 sccm.
Completing the ONO charge trapping dielectric stack 306, the blocking layer 304C of the SONOS device 300 may be any commonly known material with any thickness suitable to maintain a barrier to charge leakage without significantly decreasing the capacitance of the gate stack. In one embodiment, blocking layer 304C comprises a dielectric layer having a higher dielectric constant than silicon nitride which may include, but is not limited to, hafnium oxide, zirconium oxide, hafnium silicate, hafnium oxy-nitride, hafnium zirconium oxide and lanthanum oxide. In another embodiment, the blocking layer 304C is silicon dioxide layer, silicon oxynitride layer, or a silicon dioxide and silicon nitride stack, with a physical thickness between about 3.0 nm and about 5.0 nm.
Blocking layer 304C can be formed by any suitable means including, for example, thermal oxidation or deposition with CVD techniques. In a preferred embodiment, the blocking layer is a deposited film formed with a high-temperature CVD process. Generally, the deposition process involves providing a silicon source, such as SiH4, SiH2Cl2, or SiCl4 and an oxygen-containing gas, such as O2 or N2O in a deposition chamber at a pressure of from about 50 mT to about 1000 mT, for a period of from about 10 minutes to about 120 minutes while maintaining the substrate at a temperature of from about 650° C. to about 850° C. Preferably, the blocking layer is deposited sequentially in the same processing tool employed to form the charge trapping layer(s) 304B. More preferably, the blocking layer is formed in the same processing tool as is both the charge trapping layer(s) 304B and the tunneling layer 304A without removing the substrate between operations.
With the ONO charge trapping dielectric layers 304A, 304B and 304C formed, they are then patterned into the ONO charge trapping dielectric stack 306 in the SONOS device 300, as depicted in FIG. 3B. Conventional lithography and etching techniques may be employed to remove the charge trapping dielectric layers from other regions of the substrate, such as the HV MOS region 350 and MOS region 370. In a particular embodiment, a combination of dry and wet etch is performed to achieve a good stack sidewall profile. In one such embodiment, an inorganic spin-on anti-reflective coating (ARC), the blocking layer 304C, and the dielectric layers 304A and 304B are dry etched, with the dry etch process stopping on the sacrificial dielectric layer 303. In a subsequent wet etch operation, an etchant, such as BOE, is employed to clear sacrificial dielectric layer 303. Here too, as discussed elsewhere herein in reference to opening of window 305 of FIG. 3A, the isotropic wet etch may undercut the masked region. In this instance, undercutting the ONO charge trapping dielectric stack 306, as denoted by the dashed line in FIG. 3B, reduces the overlap between ONO charge trapping dielectric stack 306 and the sacrificial dielectric layer 303 to the amount D2. If the overlay of the ONO charge trapping dielectric stack 306 and the window 305 is not adequate (e.g. D2 becomes zero) the substrate region of SONOS device 300 may be rendered non-functional by subsequent processes. Thus, for this reason too, the dimensions and alignment of window 305 and ONO charge trapping dielectric stack 306 are important.
Upon completion of module 210, the method of FIG. 2A proceeds to module 212, wherein the substrate 302 is cleaned of organic residues left on wafer from photoresist strip or etch by-products in preparation for the formation of a gate insulator layer in the HV MOS region 350 and MOS region 370. In the particular embodiment depicted in FIG. 2A, module 212 includes a non-HF gate insulator pre-clean. Hydrofluoric acid (HF) cleans, while conventionally performed in logic CMOS processes to remove any native or chemical oxides from the substrate 302 prior to forming a gate insulator, are disadvantageous when non-volatile charge trapping dielectric layers have already been formed and remain substantially unprotected.
Conventional HF-based gate insulator pre-cleans will etch or otherwise degrade the quality of the ONO charge trapping dielectric stack 306, particularly when the stack includes a CVD formed blocking layer 304C. Therefore, in the depicted embodiment, module 212 includes cleaning operations which are substantially free of HF. For such embodiments, the substrate 302 may retain a native or chemical oxide after the cleaning operations employed in the module 212. It should be appreciated, that this concern of HF-based cleans attacking thin and critical dielectric layers is not present for standard logic MOS processes and is also not to be found in flash memory processes that protect such layers (e.g. tunnel oxide layer of flash memory device), with a polysilicon floating gate layer prior to the HF-based MOS gate insulator pre-clean.
The non-HF pre-clean of module 212 may include cleaning regimes known in the art to remove organic residues, such as, but not limited to piranha cleans, ozone cleans, and plasma cleans comprising O2 or forming gas. The non-HF pre-clean may also include a RCA Standard Clean 1 (SC1) clean comprising a mixture of water, hydrogen peroxide and ammonium hydroxide (H2O:H2O2:NH4OH). It will be appreciated that a blocking layer 304C formed by CVD may be particularly susceptible to dielectric etchants because, for example, a deposited oxide blocking layer is typically of poorer quality than a thermally formed oxide layer. The poorer quality, be it from film stress, porosity, stoichiometry or otherwise, is associated with elevated etch rates relative to thermally grown dielectric layers. Therefore, the processes employed in the pre-clean module 212 should not be too aggressive.
For example, SC1 mixtures in logic CMOS are typically employed at a ratio of 5:1:1 H2O:H2O2:NH4OH, however it has been found that this chemistry may etch a CVD silicon dioxide blocking layer 304C at an average rate of approximately 0.2 to 0.3 nm/minute. Perhaps more of a concern than this nominal etch rate, is the capacity for the SC1 chemistry to roughen the blocking layer 304C. This roughness may be characterized with RMS roughness measurements. Also a concern associated with the SC1 chemistry is formation of pinhole defects in the top oxide which may be found at a low enough density to remain undetectable with RMS roughness measurements but nonetheless decrease the quality of the blocking layer 304C. It has been found these difficulties are avoidable or at least substantially mitigated by employing an ultra-dilute SC1 clean at module 212. An ultra-dilute SC1 is substantially more dilute than 5:1:1. For example, in one advantageous embodiment the ultra-dilute SC1 comprises approximately 0.001% NH4OH, and 0.1% H2O2 in H2O. Following the ultra-dilute SC1, the non-HF pre-clean depicted in module 212, may further include an RCA Standard Clean 2 (SC2) clean comprising a mixture of H2O:H2O2:HCl in a ratios known in the art.
In an alternative embodiment, the non-HF pre-clean of module 212 may include an ozonated water cleaning regime. For such an embodiment, the SC1 clean may be replaced by the ozonated water to remove the organics and etch residues. With elimination of the SC1 clean, few metals will be left on the substrate surface and the SC2 is therefore unnecessary.
Following the non-HF pre-clean of module 212, a logic MOS gate insulator layer may be formed on the substrate 302. The logic MOS gate insulator layer may comprise any of the dielectric materials described elsewhere herein for any of the charge trapping dielectric layers 304A, 304B and 304C, but in a particular embodiment, includes a thermally grown oxide as the gate insulator layer 314 of FIG. 3C. For particular embodiments employing a silicon substrate 302, the gate insulator layer 314, thermally grown, comprises silicon oxygen bonds. As noted elsewhere herein, the integrated process flow depicted in FIG. 1 advantageously sequences the formation of the ONO charge trapping dielectric stack 306 prior to formation of the logic MOS gate insulator so that formation of the logic MOS gate insulator with a thermal process has the advantage of additionally serving to reoxidize the ONO charge trapping dielectric stack 306. Reoxidation of the blocking layer 304C may have the effect of densifying a CVD formed blocking oxide layer and improving the quality of the blocking oxide and thereby improving non-volatile charge trap memory device performance (e.g. reduced back injection). The reoxidation of the blocking layer 304C is depicted by the addition of field lines in FIG. 3C. In a further embodiment, formation of the gate insulator layer 314 may further oxidize or reoxidize a portion or all of the charge trapping layer, such as a portion or all of the charge trapping layer 304B shown in FIG. 3C, to achieve a graded band gap in the charge trapping layer 304B. Such a graded band gap may further improved non-volatile charge trap memory device performance. Reoxidation for this purpose after the deposition of the blocking layer 304C may enable a more controlled diffusion of oxidizer to controllably oxidize or reoxidize the thin charge trapping layer 304B.
Generally, the formation of the gate insulator layer 314 may include any conventional gate oxidation process whereby the substrate 302 is heated in the presence of an oxidizing gas such as, oxygen (O2), nitrous oxide (N20), nitric oxide (NO), ozone (O3), and steam (H2O). In one embodiment, the gate oxidation process is performed at a higher temperature than the temperature at which the blocking layer 304C is deposited. In a particularly advantageous embodiment, a dilute wet oxidation is employed to form the gate insulator layer 314. The dilute wet oxidation is distinct from a conventional wet oxidation in that the H2:O2 ratio is between 1 and 1.3. In one specific embodiment, a dilute oxidation with an H2:O2 ratio of approximately 1.2 is performed at a temperature of between 800° C. and 900° C. In a further embodiment, the duration of the dilute oxidation is sufficient to grow between 5.0 nm and 15.0 nm of silicon dioxide where substrate 302 is silicon. In one such embodiment, the duration is sufficient to for an approximately 10 nm to 1.1 nm silicon dioxide layer to be formed on a silicon substrate. Such a dilute wet oxidation process advantageously reoxidizes a deposited blocking layer 304C and may further oxidize or reoxidize a portion of the charge trapping layer 304B.
Where a native oxide or a chemical oxide remains on the substrate 302 after the pre-clean module 212, a thermal oxidation forms a gate insulator layer 314 comprising silicon dioxide by consuming some of the silicon below the native or chemical oxide in the substrate 302. Therefore, where multiple MOS gate insulator layer thicknesses are to be employed, for example one thickness in the region for HV MOS transistor 350 and a second thickness in the region of MOS transistor 370, it may be advantageous to form the thickest gate insulator layer at module 214 of FIG. 2A prior to additional gate insulator layers of lesser thickness so that any native or chemical oxide formed since the formation of the ONO charge trapping dielectric stack 306 is completely consumed and the electrical impact of the native oxide on the resulting MOS device is reduced by the relatively greater gate insulator layer thickness.
In one embodiment depicted in FIG. 2A, if another gate insulator layer of differing composition and/or thickness is to be formed in the process, then the method 200 proceeds to module 218. At module 218 a photoresist layer 318 of FIG. 3D is deposited and patterned to have an opening 319 formed over a region of the substrate 302 that is to have the next insulator material and/or insulator layer thickness. At module 222, any previously formed gate insulator layers, such as gate insulator layer 314, depicted in FIG. 3D, are selectively removed to expose the substrate 302. Conventional lithography and etch techniques may be employed at modules 218 and 222, such as those described in reference to module 205.
Following the removal of the gate insulator layer(s), a pre-clean may be performed on the substrate 302. For example, in the module 224, while the photoresist layer 318 protects the ONO charge trapping dielectric stack 306 a clean which would be detrimental to the ONO charge trapping dielectric stack 306 if it were not protected by the photoresist layer 318 may be performed at this time. While in certain embodiments the clean in module 224 is not performed, in either case, the photoresist layer 318 is stripped at module 226, for example with conventional piranha clean and/or plasma ash operations, subsequent to the selective removal of the gate insulator layer(s).
With the ONO charge trapping dielectric stack 306 patterned and the gate insulator layer 314 patterned, the non-HF pre-clean module 212 may then be repeated in preparation of forming another gate insulator layer. Any of the processes described elsewhere herein for module 212, such as an ultra-dilute SC1 clean, may be performed at this time to clean the substrate 302 in preparation for formation of an additional gate insulator layer in the opening 319. Following module 212, another gate insulator layer may be formed at module 214, such as gate insulator layer 320. Gate insulator layer 320 may be any of the materials described in reference to gate insulator layer 314 and not necessarily the same material as gate insulator layer 314. In one particular embodiment, gate insulator layer 320 is a thermally grown layer comprising silicon dioxide. In a further embodiment, the gate insulator layer 320 is formed over a third region of the substrate 302, such for MOS transistor 370, and is thinner than the gate insulator layer 314 formed over a second region of the substrate 302, such as for HV MOS transistor 350. In one such embodiment, the gate insulator layer 320 comprising silicon dioxide is formed to a thickness between approximately 3.0 nm and 8.0 nm while the gate insulator layer 314 is between 5 and 15 nm. Any of the processes described elsewhere herein for module 214 in reference to the formation of gate insulator layer 314 may also be employed to form the gate insulator layer 320. Additionally, the blocking layer 304C and charge trapping layer 304B may be reoxidized during the formation of the gate insulator layer 320, much as described in reference to the formation of the gate insulator layer 314. It should be appreciated such as reoxidation may be to a lesser extent than what occurs during the formation of the gate insulator layer 314, particularly where the gate insulator layer 320 is formed thinner than the gate insulator layer 314 or where the gate insulator layer 320 is formed with a process other than the dilute steam oxidation described for one embodiment of the gate insulator layer 314.
If desired, modules 218, 222, 224, 226, 212 and 214 may be repeated any number of times to provide more than the two gate insulator layer thicknesses described in the embodiment depicted in FIGS. 3C-3E. In this manner, successively thinner gate insulator layers may be formed with each iteration. For example, a third gate insulator layer may be formed to between 2.0 nm and 3.5 nm, thinner than the gate insulator layer 314 and thinner than the gate insulator layer 320.
After forming at least one MOS gate insulator layer, such as the gate insulator layer 314, the embodiment depicted in FIG. 2A proceeds to module 228. At module 228, the gate insulator layer 314 and the ONO charge trapping dielectric stack 306 are nitrided or nitridized. Beyond nitriding the MOS gate insulator, this nitridation process serves to incorporate nitrogen into the ONO charge trapping dielectric stack 306 and improve the quality of the interfaces in the stack (e.g. between the dielectric layers 304C and 304B). This nitriding process, in certain embodiments, may incorporate approximately 4-10 wt % nitrogen into the blocking layer 304C. In a particular embodiment, the nitridation process includes heating substrate 302 in an atmosphere including nitrogen at a temperature approximately in the range of 900-1100° C.
In one embodiment, nitridation of the ONO charge trapping dielectric stack 306 is performed as part of forming the gate insulator layer (e.g. gate insulator layer 314 or gate insulator layer 320). Thus, modules 214 and 228 of FIG. 2A need not be performed in separate process equipment, but rather merely a separate step of a single process recipe. This nitriding process, in certain embodiments, may incorporate approximately 4-10 wt % nitrogen into the blocking layer 304C and approximately 4-10 wt % nitrogen into the gate insulator layer 314 and/or gate insulator layer 320. In one such embodiment, a CVD furnace is employed for the nitridation of module 228 and the duration of the nitridation may be for between 5 minutes and 10 minutes. In another embodiment, a single wafer tool may be employed for the nitridation of module 228, exposing the gate insulator layer(s) and ONO charge trapping dielectric stack 306 to a nitrogen-containing environment for a duration in the range of approximately 30 seconds to approximately 60 seconds.
In an embodiment, the atmosphere including nitrogen is composed of a gas such as, but not limited to nitrogen (N2), nitrous oxide (N2O), nitrogen dioxide (NO2), nitric oxide (NO) and ammonia (NH3). In still other embodiments, the nitrogen environment further includes deuterium through an introduction of gases in which hydrogen has been replaced by deuterium, including, for example, the substitution of ND3 for NH3. The substitution of deuterium for hydrogen may advantageously passivate Si dangling bonds at the substrate interface, thereby increasing non-volatile charge trap memory device parametrics, such as NBTI (Negative Bias Temperature Instability) lifetime.
In another particular embodiment, nitridation in module 228 is performed only once after the last gate insulator layer is formed, for example, after gate insulator layer 314 and gate insulator layer 320 have been formed. The single nitridation process therefore nitridizes the ONO charge trapping dielectric stack 306, the gate insulator layer 314 and the gate insulator layer 320, as depicted by the field lines of FIG. 3F. In such an embodiment, the single nitridation provides the benefits described herein while minimizing the thermal budget of the integrated process 200.
In another embodiment, the nitridation process of module 228 is performed only once after the first gate insulator is formed, for example, after gate insulator layer 314. In such an embodiment, the ONO charge trapping dielectric stack 306 is nitrided along with the gate insulator layer 314. Nitridation only after the first gate insulator may allow for some of the logic MOS devices fabricated on substrate 302, such as in the region for HV MOS transistor 350, to have a nitrided gate insulator layer while others do not, such as in the region for MOS transistor 370.
Embodiments employing nitridation immediately after the first gate insulator is formed may also improve the ability of the ONO charge trapping dielectric stack 306 to withstand exposure to a subsequent HF-based clean performed prior to forming a subsequent gate insulator layer, such as gate insulator layer 320. In one particular embodiment, a silicon dioxide gate insulator layer 314 is formed to a thickness of approximately 5.0 nm to 15.0 nm (consuming silicon below a native oxide on the substrate 302), the nitridation of module 228 is performed, a dilute HF-based clean is performed with the nitrided ONO charge trapping dielectric stack 306 exposed and then a silicon dioxide gate insulator layer 320 of a thickness between 2.0 and 8.0 nm is formed without consuming any significant thickness of native or chemical oxide on the substrate 302. In this particular embodiment, the nitridation operation of module 228, which may be part of a thick MOS gate insulator formation process, enables a dilute HF-based pre-clean chemistry to be employed for the more critical thin MOS gate insulator formation with minimal detrimental effect on the ONO charge trapping dielectric. In still other embodiments, each successive gate insulator formed includes the nitridation process of module 228 such that the ONO charge trapping dielectric stack 306 is exposed to a plurality of nitrogen anneals.
Following the nitridation of module 228 in FIG. 2A, the method 200 proceeds with module 230. At module 230, a gate layer 330 is deposited on both the ONO charge trapping dielectric stack 306 and on the gate insulator layers 314 and 320, as depicted in FIG. 3G. The gate layer may be formed with any process conventionally known in the art. The gate layer may be any conductor or semiconductor material employed for gate layers in the art. In one embodiment, the gate layer 330 contains a metal, such as, but not limited to, hafnium, zirconium, titanium, tantalum, aluminum, ruthenium, palladium, platinum, cobalt and nickel, their silicides, their nitrides and their carbides.
In another embodiment, the gate layer 330 is poly-silicon (p-silicon). In a further embodiment, the poly-silicon gate layer 330 may be dual-doped to have N+ conductivity over a first portion of the first and/or second gate insulator layer (314, 320) to form a HV NMOS and/or NMOS transistor, respectively, while having P+ conductivity over a second portion of the first and/or second gate insulator layer (314, 320) to form a HV PMOS and/or PMOS transistor, respectively. In a further embodiment, the poly-silicon gate layer 330 may be doped to have either N+ or P+ conductivity in the SONOS device 300. Thus, the dual-doped poly-silicon may form a P+ poly-silicon gate on an N-type SONOS memory device. Because the P+ poly-silicon gate has a Fermi level approximately 1 eV higher that an N+ poly-silicon gate, the larger workfunction of a P+ poly-silicon gate on an N-type SONOS device channel may improve reliability by reducing the number of charge carriers entering the ONO charge trapping dielectric stack 306 relative to an N-type SONOS device having an N+ poly-silicon gate.
As further shown in FIG. 3H, a SONOS gate electrode is patterned over the substrate region of SONOS device 300, while a HV MOS gate electrode 358 and MOS gate electrode 378 are patterned over the HV MOS transistor 350 and MOS transistor 370 substrate regions, respectively. In a particular embodiment, the patterning of SONOS gate electrode is performed with a dry etch stopping on the ONO charge trapping dielectric stack to protect the substrate semiconductor of the SONOS device 300.
Conventional tip and/or HALO implant process may then be performed at module 255 of FIG. 2A to form lightly doped drains (not pictured). Subsequently, at module 261 and as further depicted in FIG. 3I, a sidewall spacer 309 is then formed adjacent to a sidewall of the SONOS gate electrode 308 and on the ONO charge trapping dielectric stack 306. Sidewall spacer 309, for example, may be comprised of silicon dioxide, silicon oxynitride, or silicon nitride and may also be patterned selectively to the ONO charge trapping dielectric stack 306. The ONO charge trapping dielectric stack 306 may then be subsequently etched to be self-aligned with sidewall spacer 309 to complete the formation of a SONOS gate stack 301 as depicted in FIG. 3J. Similar processes may also form spacers 359 and 379 adjacent to HV MOS gate stack 351 and a MOS gate stack 371, respectively.
FIG. 4A illustrates a cross-sectional side view of devices following the source/drain implant module 263 of FIG. 2B. Illustration of gate stacks 301, 351 and 371 is simplified relative to those of FIG. 3J merely for clarity. SONOS device 300 now includes source and drain 410 in substrate 302 having a conductivity opposite to the channel region. For example, in accordance with an embodiment of the present invention, source and drain 410 are N-type doped while channel region of substrate 302 is P-type doped. In one embodiment, substrate 302 is comprised of boron-doped single-crystal silicon having a boron concentration in the range of 1×1015-1×1019 atoms/cm3. In another embodiment, source and drain 410 are comprised of phosphorous- or arsenic-doped regions having a concentration of N-type dopants in the range of 5×1016-1×1020 atoms/cm3. In a specific embodiment, source and drain 410 have a depth in substrate 302 in the range of 80-200 nanometers. In accordance with an alternative embodiment of the present invention, source and drain 410 are P-type doped while the channel region of substrate 302 is N-type doped. As further shown, lightly doped source and drain (LDD) 411, formed at module 255 of FIG. 2A, extend under sidewall spacer 309.
Both the HV MOS transistor 350 and MOS transistor 370 also include an LDD 461 and 481, respectively. MOS transistor 370 further includes a source and drain 480 adjacent to the sidewall spacer 379 and a distance T1 away from below the sidewall of the gate stack 371. The source and drain 480 has an n-type conductivity and may, in certain embodiments, have substantially the same dopant concentration as that of the source and drain 410. However, as depicted in FIG. 4A, at module 263, the HV MOS transistor 350 lacks a source and drain analogous to the source and drain 410 and 480. Thus, during implantation of the source and drain 410 and 480, the HV MOS transistor 350 may be masked with a photosensitive mask or with a commonly known non-photosensitive hardmask, such as, but not limited to, amorphous carbon, that was previously patterned.
A multi-layered liner is then formed at module 264 of FIG. 2B. As depicted in FIG. 4B, a liner 485 is formed over the substrate 302, covering the SONOS device 300, the HV MOS transistor 350 and MOS transistor 370. In the particular embodiment depicted, the liner 485 is a multi-layered liner comprising a bottom liner layer 485A and a top liner layer 485B. The bottom liner layer 485A and top liner layer 485B may be any commonly employed materials. In a preferred embodiment, the top liner layer 485B may be anisotropically etched selectively to the bottom liner layer 485A. In one such embodiment, the bottom liner layer 485A is silicon dioxide while the top liner layer 485B is a silicon nitride. In an alternate embodiment, the bottom liner layer 485A is a silicon nitride while the top liner layer 485B is silicon dioxide. Other embodiments may include a top or bottom layer of silicon oxy-nitride, carbon-doped silicon nitride or boron-doped silicon nitride. While the multi-layered liner 485 has particular integration advantages discussed elsewhere herein, certain embodiments may also utilize a single layer liner comprised of, for example, a silicon dioxide layer or a silicon nitride layer.
A thin bottom liner layer 485A advantageously reduces the lateral thickness deposited on the sidewalls of the sidewall spacer 309, 359 and 379, which may be in close proximity to sidewalls of other devices. A thin bottom liner layer 485A may further reduce the amount of thickness variation in the film across different regions of the substrate 302, the advantage of which is discussed elsewhere herein. In one such embodiment, a silicon nitride bottom liner layer 485A is formed to a thickness of between about 2 nm about 15 nm, preferably between about 5 nm and about 8 nm. The thickness of the top liner layer 485B may be selected to provide the multi-layered liner with a desired thickness on the sidewall of the spacer 359, as discussed further elsewhere herein. In one embodiment, a silicon dioxide top liner layer 485B is formed to a thickness of between about 10 nm and 40 nm, preferably between about 20 nm and 30 nm.
The layers of the multi-layered liner 485 may be deposited with any commonly known techniques, such as, but not limited to, thermal oxidation, low pressure CVD (LPCVD) plasma enhanced CVD (PECVD) and ALD processes known to those of skill in the art. For example, a nitride bottom liner layer 485A may be deposited with a nitrogen precursor, such as NH3, and a silicon precursor, such as silane (SiH4), dichlorosilane (SiH2Cl2), or bis(tertiary-butylamino)silane (BTBAS). The deposition may be performed at a substrate temperature, for example between approximately 550° C. and approximately 850° C., and at a deposition chamber pressure between approximately 100 millitorr (mT) and approximately 700 mT, to form a film having a thickness anywhere within the ranges previously described.
An oxide layer top liner layer 485B may be similarly formed by thermal or chemical oxidation of the bottom liner layer 485A or a deposition process, such as an LPCVD employing any commonly known precursors, to form a film having a thickness anywhere within the ranges previously described. In a particular embodiment, either or both of the bottom liner layer 485A and top liner layer 485B may be deposited with techniques known to result in highly stressed films. Such stressed-film embodiments may make subsequent removal of either liner layer significantly faster and/or more selective to each other or underlying layers.
At module 265 and as further depicted in FIG. 4C, the top liner layer 485B is anisotropically etched to form disposable sidewall spacer 486 along sidewalls of topography present under the multi-layered liner 485. The top liner layer 485B is etched selectively to the bottom liner layer 485A (i.e. the bottom liner layer 485A provides an etch stop). In one such embodiment, the bottom liner layer 485A remains a substantially continuous film over the substrate 302 after the top liner layer 485B is formed in to discrete disposable sidewall spacer 486.
The process selected to anisotropically etch the top liner layer 485B to form disposable sidewall spacer 486 is dependent on the materials chosen. In the particular embodiment employing a silicon nitride bottom layer 485A and a silicon dioxide top liner layer 485B a commonly known plasma etch process may be used, such as one with a fluorine chemistry like carbon tetrafluoride (CF4), having a high enough selectivity to silicon nitride to stop prior to etching through the bottom liner layer 485A. In an alternate embodiment employing a silicon dioxide bottom layer 485A and a silicon nitride top layer 485B, any commonly known plasma etch process may be used, such as one utilizing a fluorine-based chemistry, like nitrogen trifluoride (NF3), or one utilizing a chlorine-based chemistry.
Subsequent to the formation of the disposable sidewall spacer 486, a source and drain may be formed for the HV MOS transistor at module 267. In the embodiment further depicted in FIG. 4D, the source and drain 460 is formed with a p-type implant after a mask 498 is formed over the SONOS device 300 and MOS transistor 370. Mask 498 may be any commonly known photosensitive mask material (i.e. photoresist) or non-photosensitive mask, such as amorphous carbon, that was previously patterned. The p-type dopant may be any commonly employed in the art, such as a Boron species. Other embodiments include n-type dopants for HV NMOS transistors.
The implantation, is self-aligned to the gate stack 351 and offset from a sidewall of the gate stack 351 by a distance T2. The distance T2 is approximately equal to the sidewall thickness of the sidewall spacer 359 added to the sidewall thickness of the bottom liner layer 485A added to the sidewall thickness of the disposable sidewall spacer 486. In the embodiment shown in FIG. 4D, the distance T2 for the HV MOS transistor 350 is greater than the distance T1 for the MOS transistor 370. In this manner, the source and drain 460 is offset by the distance T2, greater than T1, to increase the length of the LDD 461. Thus, the thickness of the top liner layer 485B deposited in the operation depicted in FIG. 4B may be predetermined to provide a disposable sidewall spacer 486 with the appropriate lateral width (thickness).
The relatively greater offset represented by T2 may increase the breakdown voltage by reducing the encroachment of p-type dopant diffusion from the source and drain 460 into the channel region of the HV MOS transistor 350 during subsequent thermal processing. In the embodiment depicted, the p-type implant is made through the bottom liner layer 485A. As previously described, particular embodiments employ an advantageously thin bottom liner layer 485A to improve the uniformity of the implant profile across the substrate 302.
Following the formation of the source and drain 460, the mask 498 may be removed to expose the bottom liner layer 485A covering the SONOS device 300 and MOS transistor 370. At module 268 and as further shown in FIG. 4E, the disposable sidewall spacer 486 may then be removed. Removal of the disposable sidewall spacer 486 may advantageously increase the space between adjacent logic and nonvolatile charge trap memory devices to enable a higher packing density of devices (i.e. smaller device pitch). This is particularly advantageous for SONOS device 300 which may be part of a closely spaced array of SONOS devices, such as in a memory cell array. Removal of the disposable sidewall spacer 486 may also improve the step coverage of subsequently deposited ILD layers over high density SONOS and logic devices.
Removal of the disposable sidewall spacer 486 may be with a masked process, whereby the disposable sidewall spacer 486 is removed, for example, from the SONOS device 300, but retained on the HV MOS transistor 350. However, in the embodiment depicted, the disposable sidewall spacer 486 is removed from the entire substrate 302 with an unmasked etch process. As shown, the etch process is selective to the bottom liner layer 485A (i.e. the bottom liner layer 485A acts as an etch stop for the etch process employed to remove the disposable sidewall spacer 486. With the protection of bottom liner layer 485A, substrate semiconductor and substrate insulator layers, such as shallow trench isolation (STI) are protected from the process employed to remove the disposable sidewall spacer 486. Because it has been found that processing of the corner (not pictured, but is out of the plane of FIG. 4E) formed where the STI meets the width of the gate stack 301 can greatly effect the performance of the SONOS device 300, it is advantageous not to expose this region to the process employed for disposable spacer removal.
With the bottom liner 485A serving as an etch stop layer, the substrate 302, the SONOS gate electrode 308, the HV PMOS gate layer 358 and the NMOS gate layer 378, as well as the sidewall spacers 309, 359 and 379 remain protected during the removal of the disposable sidewall spacer 486. With such features protected, the material composition of the disposable sidewall spacer 486 (i.e. top liner layer 485B) is independent of the materials in the gate electrodes 308, 358 and 378 and the sidewall spacers 309, 359 and 379.
Disposable sidewall spacer 486 may be removed with commonly known wet chemical or dry etch processes, depending on the materials employed in the particular implementation. In one embodiment, wherein the disposable sidewall spacer 486 comprises silicon dioxide (i.e. a silicon dioxide top liner layer 485B), a hydrofluoric acid (HF) based wet chemical etch may be performed to remove the disposable sidewall spacer 486 selectively to a silicon nitride bottom liner layer 485A. In an alternate implementation, an isotropic dry etch process, such as one commonly known to have a high selectivity over the silicon nitride bottom liner layer 485A may be employed. In another embodiment, wherein the disposable sidewall spacer 486 comprises silicon nitride (i.e. a silicon nitride top liner layer 485B), a hot phosphoric acid (H3PO4) based wet chemical etch may be performed to remove the disposable sidewall spacer 486 selectively to a silicon dioxide bottom liner layer 485A. In an alternate implementation, an isotropic dry etch process, such as one commonly known to have a high selectivity over the silicon dioxide bottom liner layer 485A may be employed.
Subsequent to the removal of the disposable sidewall spacer 486, the bottom liner layer 485A may be removed at module 269 to expose the source and drain regions of either or both of the SONOS and logic devices in preparation for a silicidation or salicidation (self-aligned silicidation) process at module 270. In one embodiment, a blanket strip of the bottom liner layer 485A may be performed to expose the source and drain regions of all devices. In such an embodiment, the strip process is preferably selective to the STI corner, the gate electrodes 308, 358 and 378 and the sidewall spacers 309, 359 and 379. FIG. 4F, however, depicts an alternate embodiment utilizing a patterned etch of the bottom liner layer 485A. The patterned etch exposes only the source and drains of those devices for which silicide is desired. The bottom liner layer 485A may thereby further provide for device-dependent silicidation.
As previously discussed, because the silicidation process can induce stress, silicide may be detrimental to the performance and reliability of the SONOS device 300. Therefore, a device-dependent silicidation process may be advantageous for integrating a logic device, such as MOS transistor 370, having silicide contacts with a non-volatile charge trap memory device, such as SONOS device 300, having silicide-free contacts. Similarly, a HV MOS device, such as HV MOS transistor 350, may include either silicide or silicide-free contacts. As shown in FIG. 4E, the LDD region 461 extends beyond the sidewall spacer 359 and bottom liner 485A (i.e. below where the disposable sidewall spacer 486 was removed) and formation of silicide over this exposed LDD region may not be desirable.
In one embodiment, as shown in FIG. 4F, a mask 499 is formed over the SONOS device 300 and HV MOS transistor 350. Mask 499 may be any commonly known photosensitive mask material (i.e. photoresist) or non-photosensitive mask, such as amorphous carbon, which is first patterned. The bottom liner layer 485A may then be etched to expose the regions of the MOS transistor 370 for subsequent silicidation. Removal of the bottom liner layer 485A may be done by any commonly known means dependent on the material composition. Advantageously, the removal process should be selective to the semiconductor substrate 302 and the STI (not shown). It may further be advantageously selective to the gate layer 378, and the sidewall spacer 379. In one particular embodiment employing a silicon nitride bottom liner layer 485A, a phosphoric acid-based wet chemical etch is utilized. In an alternate embodiment employing a silicon dioxide bottom liner layer 485A, an HF-based wet chemical etch may be used. Because the thickness of bottom liner layer 485A is relatively small, the etch and overetch time may be kept short to avoid eroding underlayers. Furthermore, a wet chemical etch rate may be greatly enhanced for those embodiments previously described having a highly stressed bottom liner layer 485A. In still other embodiments, a dry plasma etch employing commonly known process parameters may also be used to remove the unmasked portion of the bottom liner layer 485A. Mask 499 may then be removed.
As shown in FIG. 4G, a silicide process may then be performed on those areas with exposed silicon. The silicide process may be any commonly employed in the art, typically including a pre-clean etch, cobalt or nickel metal deposition, anneal and wet strip. As depicted, silicide region 482 may be formed on the exposed gate layer 378 and exposed source and drain region while blocked by the bottom liner layer 485A from the SONOS device 300 and HV device (e.g. MOS transistor 350) regions.
In one embodiment, subsequent to the operations depicted in FIG. 4G, processing proceeds, as shown in FIG. 5, with a removal of the bottom liner layer 485A and deposition of ILD 504. Such an embodiment has the advantage of simplifying a subsequent contact etch because etching of ILD 504 will expose the source and drain 410, the source and drain 460 and the silicide region 482 of the MOS transistor 370. In this embodiment, the devices with silicide may be masked with any commonly known photosensitive mask material (i.e. photoresist) or non-photosensitive mask, such as amorphous carbon, which is first patterned. Masking of the silicided devices (e.g. MOS transistor 370) is advantageous if the process employed to remove the bottom liner layer 485A is nonselective to the silicide. Otherwise, a blanket strip of the bottom liner layer 485A may be performed. A backend interconnect process may then begin with a deposition of ILD 504 over non-silicided SONOS device 300, non-silicided HV MOS transistor 350 and silicided MOS transistor 370.
In an alternative embodiment shown in FIG. 6A, subsequent to the operations depicted in FIG. 4G, processing proceeds with deposition of an ILD over the bottom liner layer 485A and over the silicide region 482. In such an embodiment, the bottom liner layer 485A is incorporated as part of the backend ILD and subsequently removed during contact etch with an etch step selective over the silicide region 482. In a particular embodiment employing a silicon nitride bottom liner layer 485A, the bottom liner layer 485A may be further utilized in a self-aligned contact (SAC) etch. The SAC etch, employing an etch recipe highly selective to silicon nitride may reduce the contact dimension from that lithographically printed to the physical space between adjacent devices covered with the bottom liner layer 485A.
As further depicted in FIG. 6A, the ILD layer deposited over the bottom liner layer 485A and silicide region 482 may include a stress-inducing layer 504A. Stress-inducing layer 504A may be composed of any material and have any thickness suitable to exert a stress on channel region of a logic device. Stress-inducing layer 504A may advantageously increase the carrier mobility and drive currents of a logic device, such as MOS transistor 370. In accordance with an embodiment of the present invention, stress-inducing layer 504A is disposed directly on MOS transistor 370. In one embodiment, stress-inducing layer 504A is deposited to a thickness approximately in the range of 20-100 nanometers and is composed of a material such as, but not limited to, silicon nitride, silicon oxy-nitride, carbon-doped silicon nitride or boron-doped silicon nitride. In a specific embodiment, stress-inducing layer 504A is a tensile stress-inducing layer.
Because the stress inducing layer 504A may induce an undesirable stress in the SONOS device 300, causing performance and reliability degradation, the bottom liner layer 485A may have been deposited under conditions to induce a stress opposing that of stress inducing layer 504A. In a particular embodiment, the bottom liner layer 485A may induce a compressive stress opposing a tensile stress in the stress inducing layer 504A. In one such embodiment, the stress of the bottom liner layer 485A reduces the cumulative stress on the SONOS device 300 to an amount less than half that induced by the stress inducing layer 504A in absence of the bottom liner layer 485A. The bottom liner layer 485A may therefore provide both selective silicidation and selective stress induction for integration of the SONOS device 300 and a logic device, such as MOS transistor 370. As further shown in FIG. 4, ILD layer 504B may then be deposited over stress inducing layer 504A and planarized as part of a conventional backend interconnect process.
In still another embodiment, as depicted in FIG. 6B, the bottom liner layer 485A may provide an etch stop for removal of the stress-inducing layer 504A from over the SONOS device 300 and HV MOS transistor 350. The portion of stress-inducing layer 504A above a nonvolatile charge trap memory device (e.g. SONOS device 300) may be removed through a lithography and etch process selective to the bottom liner layer 485A. In one embodiment, the portion of stress-inducing layer 504A above MOS transistor 370 is first masked with a patterned photo-resist layer and the portion of stress-inducing layer 504A above SONOS device 300 is then removed by a technique such as, but not limited to, a wet etch process using hot phosphoric acid or a conventional dry etch process.
In embodiments where the bottom liner layer 485A is a silicon dioxide, the stress inducing layer 504A may be removed with high selectively to the bottom liner layer 485A. In other embodiments employing a silicon nitride bottom liner layer 485A, the higher stress of the stress inducing layer 504A may provide selectivity to the bottom liner layer 485A. The bottom liner layer 485A therefore may provide protection to the underlying structures during patterning of the stress inducing layer 504A. In alternate embodiments, removal of the stress inducing layer 504A also removes the bottom liner layer 485A.
As shown in FIG. 6B, if the stress inducing layer 504A is removed selectively to the bottom liner layer 485A (e.g. to prevent a stress inducing layer 504A from detrimentally impacting performance of SONOS device 300 or HV MOS transistor 350), the ILD layer 504B may then be deposited over both the bottom liner layer 485A and over the stress inducing layer 504A in preparation for further backend interconnect processing.
Thus, a semiconductor structure integrating charge trap memory devices with logic devices and method to form the same has been disclosed. Although the present invention has been described in language specific to structural features or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. The specific features and acts disclosed are to be understood as particularly graceful implementations of the claimed invention in an effort to illustrate rather than limit the present invention.

## Claims

1. A method of fabricating a semiconductor structure, comprising:
forming a nonvolatile charge trap memory device over a first portion of a substrate, the nonvolatile charge trap memory device further comprising a charge trapping dielectric stack consisting of dielectric material layers that include a blocking layer, a charge trapping layer, and a tunneling layer, wherein the tunneling layer is in contact with the first portion of the substrate;
forming an NMOS gate stack adjacent to an NMOS sidewall spacer over a second portion of the substrate;
implanting an NMOS source and drain region adjacent to the NMOS spacer prior to forming a liner;
forming a PMOS gate stack adjacent to a PMOS sidewall spacer over a third portion of the substrate prior to forming the liner;
forming the liner over the first portion of the substrate;
implanting a PMOS source and drain subsequent to forming the liner; and
forming a silicide adjacent to the NMOS sidewall spacer while the liner protects the nonvolatile charge trap memory device from silicide formation.

2. The method of claim 1, further comprising:
removing the liner over the first portion of the substrate subsequent to forming the silicide.

3. The method of claim 1, further comprising:
depositing an interlayer dielectric (ILD) layer over the first and second portions of the substrate, the ILD layer deposited over the liner in the first portion.

4. The method of claim 3, wherein the ILD layer is deposited to have a stress of opposite sign as that of the liner.

5. The method of claim 3, further comprising:
etching contact openings in the ILD over the first and second portions;
etching contact openings in the liner over the first substrate portion selectively to the silicide in the second substrate portion.

6. The method of claim 1, wherein forming the liner further comprises:
depositing a multi-layered liner over the PMOS sidewall spacers, the multilayered liner including a top layer and a bottom layer; and
etching the top layer selective to the bottom layer to form a disposable sidewall spacer separated from the PMOS sidewall spacer by at least the bottom layer.

7. The method of claim 6, further comprising:
removing the disposable sidewall spacer, selectively to the bottom layer of the multi-layered liner, subsequent to implanting the PMOS source and drain; and
removing the bottom layer of the multi-layered liner from the second portion, while protecting the bottom layer in the first portion, to expose the NMOS gate stack without exposing the nonvolatile trapped charge device.

8. The method of claim 1, wherein forming the nonvolatile charge trap memory device over the first portion further comprises:
fanning the tunneling layer on the first portion of the substrate;
forming the charge trapping layer on the tunneling layer, wherein the charge trapping layer is formed with a non-homogenous silicon concentration with a lower silicon concentration proximate to the tunneling layer;
forming the blocking layer on the charge trapping layer; and
re-oxidizing the blocking layer while forming a gate insulator layer of the NMOS gate stack.

9. A method of fabricating a semiconductor structure, comprising:
forming a nonvolatile charge trap memory device over a first portion of the substrate;
forming an NMOS gate stack adjacent to an NMOS sidewall spacer over a second portion of the substrate;
forming a PMOS gate stack adjacent to a PMOS sidewall spacer over a third portion of the substrate;
implanting an NMOS source and drain region adjacent to the NMOS sidewall spacer;
after implanting an NMOS source and drain region, depositing a multi-layer liner over the NMOS and PMOS sidewall spacers, the multi-layer liner including a top layer and a bottom layer;
etching the top layer selective to the bottom layer to form a disposable sidewall spacer separated from the NMOS sidewall spacer and the PMOS sidewall spacer by at least the bottom layer; and
implanting a PMOS source and a drain adjacent to the PMOS sidewall spacer after forming the disposable sidewall spacers to offset the implant and increase PMOS breakdown voltage.

10. The method of claim 9, wherein the bottom layer of the multi-layered liner is silicon nitride, the top layer is silicon dioxide and the disposable spacer is removed with a wet etchant comprising HF subsequent to implanting the PMOS source and drain.

11. The method of claim 9, further comprising:
removing the bottom layer of the multi-layered liner from the NMOS source and drain;
forming a silicide on the NMOS source and drain while the bottom layer of the multi-layered liner prevents formation of a silicide on the PMOS or on the nonvolatile charge trap memory device.