This application is a divisional under 37 C.F.R. 1.53(b) of U.S. Ser. No. 10/163,686 filed on Jun. 5, 2002, which application is incorporated herein by reference.
FIELD OF THE INVENTION
This application is related to the following, co-pending, commonly assigned applications, incorporated herein by reference: U.S. application Ser. No. 10/027,315 filed Dec. 20, 2001, entitled: “Low-Temperature Grown High-Quality Ultra-Thin Praseodymium Gate Dielectrics.”
- BACKGROUND OF THE INVENTION
The invention relates to semiconductor devices and device fabrication. Specifically, the invention relates to gate dielectric layers of transistor devices and their method of fabrication.
The semiconductor device industry has a market driven need to improve speed performance, improve its low static (off-state) power requirements, and adapt to a wide range of power supply and output voltage requirements for it silicon based microelectronic products. In particular, in the fabrication of transistors, there is continuous pressure to reduce the size of devices such as transistors. The ultimate goal is to fabricate increasingly smaller and more reliable integrated circuits (ICs) for use in products such as processor chips, mobile telephones, or memory devices such as DRAMs. The smaller devices are frequently powered by batteries, where there is also pressure to reduce the size of the batteries, and to extend the time between battery charges. This forces the industry to not only design smaller transistors, but to design them to operate reliably with lower power supplies.
Currently, the semiconductor industry relies on the ability to reduce or scale the dimensions of its basic devices, primarily, the silicon based metal-oxide-semiconductor field effect transistor (MOSFET). A common configuration of such a transistor is shown in FIG. 1. While the following discussion uses FIG. 1 to illustrate a transistor from the prior art, one skilled in the art will recognize that the present invention could be incorporated into the transistor shown in FIG. 1 to form a novel transistor according to the invention. The transistor 100 is fabricated in a substrate 110 that is typically silicon, but could be fabricated from other semiconductor materials as well. The transistor 100 has a first source/drain region 120 and a second source/drain region 130. A body region 132 is located between the first source/drain region and the second source/drain region, where the body region 132 defines a channel of the transistor with a channel length 134. A gate dielectric, or gate oxide 140 is located on the body region 132 with a gate 150 located over the gate dielectric. Although the gate dielectric can be formed from materials other than oxides, the gate dielectric is typically an oxide, and is commonly referred to as a gate oxide. The gate may be fabricated from polycrystalline silicon (polysilicon), or other conducting materials such as metal may be used.
In fabricating transistors to be smaller in size and reliably operating on lower power supplies, one important design criteria is the gate dielectric 140. The mainstay for forming the gate dielectric has been silicon dioxide, SiO2. A thermally grown amorphous SiO2 layer provides an electrically and thermodynamically stable material, where the interface of the SiO2 layer with underlying Si provides a high quality interface as well as superior electrical isolation properties. In typical processing, use of SiO2 on Si has provided defect charge densities on the order of 1010/cm2, midgap interface state densities of approximately 1010/cm2 eV, and breakdown voltages in the range of 15 MV/cm. With such qualities, there would be no apparent need to use a material other than SiO2, but increased scaling and other requirements for gate dielectrics create the need to find other dielectric materials to be used for a gate dielectric.
- SUMMARY OF THE INVENTION
What is needed is an alternate dielectric material for forming a gate dielectric that has a high dielectric constant relative to SiO2, and is thermodynamically stable with respect to silicon such that forming the dielectric on a silicon layer will not result in SiO2 formation, or diffusion of material, such as dopants, into the gate dielectric from the underlying silicon layer.
A solution to the problems as discussed above is addressed in embodiments according to the teachings of the present invention. In one embodiment, a method of forming a gate dielectric includes forming a layer of Pr2O3 on a substrate and forming a layer of another lanthanide oxide onto the layer of Pr2O3. This second layer is formed of a lanthanide oxide selected from the group consisting of Nd2O3, Sm2O3, Gd2O3, and Dy2O3. Further, the layer of Pr2O3 and the layer of the lanthanide oxide can be formed as a nanolaminate. Advantageously, a gate dielectric formed as a combination of layers of Pr2O3 and another lanthanide oxide has a larger dielectric constant than silicon dioxide, a relatively small leakage current, and good stability with respect to a silicon based substrate. Embodiments according to the teachings of the present invention include forming transistors, memory devices, and electronic systems.
Other embodiments include structures for transistors, memory devices, and electronic systems with dielectric gates of layers of Pr2O3 and another lanthanide oxide. Such dielectric gates provide a significantly thinner equivalent oxide thickness compared with a silicon oxide gate having the same physical thickness. Alternatively, such dielectric gates provide a significantly thicker physical thickness than a silicon oxide gate having the same equivalent oxide thickness.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims.
FIG. 1 depicts a common configuration of a transistor.
FIG. 2 depicts an embodiment of a deposition process for forming a gate dielectric using electron beam evaporation according to the teachings of the present invention.
FIG. 3 depicts an embodiment of another configuration of a transistor capable of being fabricated according to the teachings of the present invention.
FIG. 4 illustrates a perspective view of an embodiment of a personal computer incorporating devices made according to the teachings of the present invention.
FIG. 5 illustrates a schematic view of an embodiment of a processing unit incorporating devices made according to the teachings of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 6 illustrates a schematic view of an embodiment of a DRAM memory device according to the teachings of the present invention.
In the following detailed description of the invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention.
The terms wafer and substrate used in the following description include any structure having an exposed surface with which to form the integrated circuit (IC) structure of the invention. The term substrate is understood to include semiconductor wafers. The term substrate is also used to refer to semiconductor structures during processing, and may include other layers that have been fabricated thereupon. Both wafer and substrate include doped and undoped semiconductors, epitaxial semiconductor layers supported by a base semiconductor or insulator, as well as other semiconductor structures well known to one skilled in the art. The term conductor is understood to include semiconductors, and the term insulator or dielectric is defined to include any material that is less electrically conductive than the materials referred to as conductors.
The term “horizontal” as used in this application is defined as a plane parallel to the conventional plane or surface of a wafer or substrate, regardless of the orientation of the wafer or substrate. The term “vertical” refers to a direction perpendicular to the horizontal as defined above. Prepositions, such as “on”, “side” (as in “sidewall”), “higher”, “lower”, “over” and “under” are defined with respect to the conventional plane or surface being on the top surface of the wafer or substrate, regardless of the orientation of the wafer or substrate. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.
A gate dielectric 140 of FIG. 1, when operating in a transistor, has both a physical gate dielectric thickness and an equivalent oxide thickness (teq). The equivalent oxide thickness quantifies the electrical properties, such as capacitance, of a gate dielectric 140 in terms of a representative physical thickness. teq is defined as the thickness of a theoretical SiO2 layer that would be required to have the same capacitance density as a given dielectric, ignoring leakage current and reliability considerations.
A SiO2 layer of thickness, t, deposited on a Si surface as a gate dielectric will also have a teq larger than its thickness, t. This teq results from the capacitance in the surface channel on which the SiO2 is deposited due to the formation of a depletion/inversion region. This depletion/inversion region can result in teq being from 3 to 6 Angstroms (Å) larger than the SiO2 thickness, t. Thus, with the semiconductor industry driving to someday scale the gate dielectric equivalent oxide thickness, teq, to under 10 Å, the physical thickness requirement for a SiO2 layer used for a gate dielectric would be need to be approximately 4 to 7 Å.
Additional requirements on a SiO2 layer would depend on the gate electrode used in conjunction with the SiO2 gate dielectric. Using a conventional polysilicon gate would result in an additional increase in teq for the SiO2 layer. This additional thickness could be eliminated by using a metal gate electrode, though metal gates are not currently used in complementary metal-oxide-semiconductor field effect transistor (CMOS) technology. Thus, future devices would be designed towards a physical SiO2 gate dielectric layer of about 5 Å or less. Such a small thickness requirement for a SiO2 oxide layer creates additional problems.
Silicon dioxide is used as a gate dielectric, in part, due to its electrical isolation properties in a SiO2—Si based structure. This electrical isolation is due to the relatively large band gap of SiO2 (8.9 eV) making it a good insulator from electrical conduction. Signification reductions in its band gap would eliminate it as a material for a gate dielectric. As the thickness of a SiO2 layer decreases, the number of atomic layers, or monolayers of the material in the thickness decreases. At a certain thickness, the number of monolayers will be sufficiently small that the SiO2 layer will not have a complete arrangement of atoms as in a larger or bulk layer. As a result of incomplete formation relative to a bulk structure, a thin SiO2 layer of only one or two monolayers will not form a full band gap. The lack of a full band gap in a SiO2 gate dielectric would cause an effective short between an underlying Si channel and an overlying polysilicon gate. This undesirable property sets a limit on the physical thickness to which a SiO2 layer can be scaled. The minimum thickness due to this monolayer effect is thought to be about 7-8 Å. Therefore, for future devices to have a teq less than about 10 Å, other dielectrics than SiO2 need to be considered for use as a gate dielectric.
For a typical dielectric layer used as a gate dielectric, the capacitance is determined as one for a parallel plate capacitance: C=κε0A/t, where κ is the dielectric constant, ε0 is the permittivity of free space, A is the area of the capacitor, and t is the thickness of the dielectric. The thickness, t, of a material is related to teq for a given capacitance with the dielectric constant of SiO2, κox=3.9, associated with teq, as
Thus, materials with a dielectric constant greater than that of SiO2, 3.9, will have a physical thickness that can be considerably larger than a desired teq, while providing the desired equivalent oxide thickness. For example, an alternate dielectric material with a dielectric constant of 10 could have a thickness of about 25.6 Å to provide a teq of 10 Å, not including any depletion/inversion layer effects. Thus, the reduced equivalent oxide thickness of transistors can be realized by using dielectric materials with higher dielectric constants than SiO2.
The thinner equivalent oxide thickness, teq required for lower transistor operating voltages and smaller transistor dimensions may be realized by a significant number of materials, but additional fabricating requirements makes determining a suitable replacement for SiO2 difficult. The current view for the microelectronics industry is still for Si based devices. This requires that the gate dielectric employed be grown on a silicon substrate or silicon layer, which places significant restraints on the substitute dielectric material. During the formation of the dielectric on the silicon layer, there exists the possibility that a small layer of SiO2 could be formed in addition to the desired dielectric. The result would effectively be a dielectric layer consisting of two sublayers in parallel with each other and the silicon layer on which the dielectric is formed. In such a case, the resulting capacitance would be that of two dielectrics in series. As a result, the teq of the dielectric layer would be the sum of the SiO2 thickness and a multiplicative factor of the thickness, t, of the dielectric being formed, written as
t eq =t SiO2+(κox/κ)t.
Thus, if a SiO2 layer is formed in the process, the teq is again limited by a SiO2 layer. In the event that a barrier layer is formed between the silicon layer and the desired dielectric in which the barrier layer prevents the formation of a SiO2 layer, the teq would be limited by the layer with the lowest dielectric constant. However, whether a single dielectric layer with a high dielectric constant or a barrier layer with a higher dielectric constant than SiO2 is employed, the layer interfacing with the silicon layer must provide a high quality interface to maintain a high channel carrier mobility.
One of the advantages using SiO2 as a gate dielectric has been that the formation of the SiO2 layer results is an amorphous gate dielectric. Having an amorphous structure for a gate dielectric is advantageous because grain boundaries in polycrystalline gate dielectrics provide high leakage paths. Additionally, grain size and orientation changes throughout a polycrystalline gate dielectric can cause variations in the film's dielectric constant. The abovementioned material properties, including structure, are for the materials in a bulk form. Many materials having the advantage of a high dielectric constant relative to SiO2 also have the disadvantage of a crystalline form, at least in a bulk configuration. The best candidates for replacing SiO2 as a gate dielectric are those with high dielectric constant, which can be fabricated as a thin layer with an amorphous form.
One candidate for forming gate dielectrics is Pr2O3. In co-pending, commonly assigned U.S. patent applications: entitled “Low-Temperature Grown High-Quality Ultra-Thin Praseodymium Gate Dielectrics,” attorney docket no. 1303.033US1, Ser. No. 10/027,315, Pr2O3 is disclosed as a replacement for SiO2 in forming gate dielectrics and other dielectric films in electronic devices such as MOS transistors. Additionally, in a recent article by H. J. Osten et al., Technical Digest of IEDM, pp. 653-656 (2000), crystalline praseodymium oxide on silicon was reported to have outstanding dielectric properties.
However, in a recent article by H. Zhang et al., Journal of the Electrochemical Societ, 148 (4) pp. F63-F66 (2001), it was noted that dielectric layers using high-κ materials tend to have a narrower bandgap. The article reported investigating the use of nanolaminates of ZrO2/HfO2, ZrO2/Ta2O5, and Ta2O5/HfO2, instead of a single layer of either Ta2O5, ZrO2, or HfO2. Each nanolaminate, that is, a composite of thin alternating layers of insulators, was grown using Atomic Layer Deposition. The dielectric layers formed of these nanolaminates were reported to have a dielectric constant in the range of 9 to 16, providing a teq reduction factor of about 3 relative to SiO2.
Embodiments according to the teachings of the present invention provide a novel set of dielectric structures for replacing SiO2 as a gate dielectric and as other dielectrics requiring an ultra-thin equivalent oxide thicknesses, teq. Dielectric layers containing layers of Pr2O3 and another lanthanide oxide for use as the replacement dielectric are formed in various embodiments. In one embodiment, a dielectric layer is grown by forming a layer of Pr2O3 on a substrate and forming a layer of another lanthanide oxide onto the layer of Pr2O3. In another embodiment, forming a layer of Pr2O3 on a substrate and forming a layer of another lanthanide oxide onto the layer of Pr2O3 is controlled to form a thin layer of each material with the combination of the two alternating layers of insulators forming a nanolaminate. The other lanthanide oxide used to form the nanolaminate is selected from a group consisting of Nd2O3, Sm2O3, Gd2O3, and Dy2O3. Alternately, the nanolaminate formed as the dielectric layer can be formed from multiple alternating thin layers of lanthanide oxides with the initial layer deposited being a layer of Pr2O3. Advantageously, using Pr2O3 for the initial layer provides a thin amorphous layer or region at the surface of a silicon based substrate.
By forming the dielectric layer as a nanolaminate, the dielectric layer can be grown as a composite whose insulating properties can be adjusted. The engineering of the nanolaminate allows the formation of a dielectric layer with a dielectric constant ranging between the values of the dielectric constants of the lanthanide oxides which form the dielectric layer. However, in many cases the dielectric layer will have an effective dielectric constant less than the dielectric constant of the lanthanide oxides used to form the dielectric layer. This reduction in the effective dielectric constant is due to interfacial layers formed between the silicon substrate surface and the first lanthanide oxide layer of the nanolaminate.
In one embodiment, a nanolaminate is grown by forming a layer of Pr2O3 on a substrate by electron beam evaporation. Subsequently, a layer of another lanthanide oxide is formed onto the layer of Pr2O3 also by electron beam evaporation. The resulting nanolaminate has a total thickness which is about the same as the thickness of the initial layer of Pr2O3 formed on the substrate. Thus, each layer in the nanolaminate is reduced to one-half the original thickness deposited. Such films are uniformly produced with a teq less than 20 Å, typically with about a teq of about 14 Å.
FIG. 2 depicts an embodiment of a deposition process for forming a gate dielectric using electron beam evaporation according to the teachings of the present invention. This process can be used to deposit a material forming a film containing a layer of Pr2O3 and a layer of another lanthanide oxide on a surface such as a body region of a transistor. In FIG. 2, a substrate 210 is placed inside a deposition chamber 260. The substrate in this embodiment is masked by a first masking structure 270 and a second masking structure 271. In this embodiment, the unmasked region 233 includes a body region of a transistor, however one skilled in the art will recognize that other semiconductor device structures may utilize this process. Also located within the deposition chamber 260 is an electron gun 263 and a target 261. The electron gun 263 provides an electron beam 264 directed at target 261 containing a source material for forming Pr2O3 and other lanthanide oxides on the unmasked region 233 of the substrate 210. The electron gun 263 includes a rate monitor for controlling the rate of evaporation of the material in the target 261 at which the electron beam 264 is directed. For convenience, control displays and necessary electrical connections as are known to those skilled in the art are not shown in FIG. 2. Alternatively, a chamber can be used with multiple electron guns, where each electron gun is directed to different targets containing sources to form selected lanthanide oxides to be used at different times in the process.
During the evaporation process, the electron gun 263 generates an electron beam 264 that hits target 261. In one embodiment, target 261 contains a ceramic Pr6O11 source, which is evaporated due to the impact of the electron beam 264. The evaporated material 268 is then distributed throughout the chamber 260. A dielectric layer of Pr2O3 is grown forming a film 240 on the surface of the exposed body region 233 that it contacts. The growth rate can vary with a typical rate of 0.1 Å/s. The resultant Pr2O3 layer includes a thin amorphous interfacial layer of about 0.5 nm thickness separating a crystalline layer of Pr2O3 from the substrate on which it is grown. This thin amorphous layer is beneficial in reducing the number of interface charges and eliminating any grain boundary paths for conductance from the substrate. Other source materials can be used for forming the Pr2O3 layer, as are known to those skilled in the art.
Subsequent to the formation of the Pr2O3 layer, another lanthanide oxide is deposited on the film 240 converting the film 240 from a Pr2O3 layer to a nanolaminate of Pr2O3 and the other lanthanide oxide. The other lanthanide oxide is selected from the group consisting of Nd2O3, Sm2O3, Gd2O3, and Dy2O3. Depending on the lanthanide oxide selected to form the nanolaminate, a corresponding source material is used in the target 261 for electron beam evaporation. The source material for the particular lanthanide oxide is chosen from commercial materials for forming the lanthanide oxide by electron bean evaporation, as is known by those skilled in the art.
After forming the nanolaminate, the structure is annealed briefly at 600° C. As a result of this brief anneal, there is no significant hysteresis in capacitance-voltage (C-V) measurements. Further, the nanolaminates can be annealed up to 1000° C. for 15 seconds with no degradation in electrical properties. In one embodiment, such films have a teq of 14 Å with a leakage current of approximately 5×10−9 Å/cm2 at a gate voltages of ±1 V, with a dielectric constant ranging from the dielectric constant of a Pr2O3 film on silicon, 31, to the dielectric constant of the other selected lanthanide oxide. Advantageously, this leakage current is at least 104 times lower that the best published value of HfO2 or ZrO2 films with the same teq and a 3 nm thick SiO2 layer.
In one embodiment alternating layers of Pr2O3 and another selected lanthanide oxide are formed by controlled electron beam evaporation providing layers of material of predetermined thickness. This control allows the engineering of a dielectric with a predetermined thickness, and composition. Through evaluation of different lanthanide oxides at various thicknesses and number of layers, a dielectric layer with a predetermined teq in a narrow range of values can be grown. Alternately, after forming a Pr2O3 layer and a layer of another lanthanide oxide, additional layers of additional lanthanide oxides can be formed. Each layer of an additional lanthanide oxide selected from a group consisting of Pr2O3, Nd2O3, Sm2O3, Gd2O3, and Dy2O3. Consequently, a dielectric layer can be engineered with electrical characteristics suited for a given application. These electrical characteristics include teq and leakage current. A teq of less than 20 Å can be obtained with typically sizes about of about 14 Å to 8.5 Å.
In another embodiment, nanolaminates of lanthanide oxides are formed by electron beam evaporation. The lanthanide oxides used in these nanolaminates are chosen from the group consisting of Pr2O3, Nd2O3, Sm2O3, Gd2O3, and Dy2O3. The structure of the nanolaminates can be varied with any one of the group used as the initial layer formed on a substrate. Typically, the substrate is silicon based, since these lanthanide oxides are thermodynamically stable with respect to formation on a silicon surface. In an alternate embodiment, lanthanide oxide nanolaminates are formed by atomic layer deposition.
The Pr2O3 film formed on a silicon has a dielectric constant of about 31 when formed with little or no interfacial layer between the Pr2O3 film and the substrate. The dielectric constants for the other lanthanide oxides are also in the range of 25-30. As a result, a dielectric layer grown by forming a nanolaminate of lanthanide oxides has a dielectric constant in the range of about 25 to about 31. However, with an interfacial layer formed between the surface of the substrate and the first lanthanide oxide, the teq of the dielectric layer is the teq of the interfacial layer in parallel with the lanthanide oxide nanolaminate. Thus, the dielectric layer formed having an interfacial layer between the substrate on which it is grown and a lanthanide oxide nanolaminate can have an effective dielectric constant considerably less than a dielectric constant associated with a nanolaminate of lanthanide oxides. This is dependent upon the dielectric constant of the interfacial material being considerably less than the dielectric constant of the lanthanide oxides used to form the nanolaminate.
As previously noted above, a Pr2O3 layer can be formed on a silicon based substrate having a dielectric constant of about 31 with an interfacial layer of about 0.5 nm (5 Å). In another embodiment, for an interfacial layer of about 10.7 Å, an effective dielectric constant for a thin layer of Pr2O3 on silicon is about 15. Similar effective dielectric constants are associated with thin layers of Nd2O3, Sm2O3, Gd2O3, and Dy2O3 oxides on silicon. For example, a thin layer of Nd2O3 has an effective dielectric constant of about 12.9 with an interfacial layer of about 8.2 Å, a thin layer of Sm2O3 has an effective dielectric constant of about 11.4 with an interfacial layer of about 5.5 Å, a thin layer of Gd2O3 has an effective dielectric constant of about 13.9 with an interfacial layer of about 10 Å, and a thin layer of Dy2O3 has an effective dielectric constant of about 14.3 with an interfacial layer of about 12 Å. Lanthanide oxides grown on silicon with these reduced effective dielectric constants and corresponding interfacial layers can be attained with a teq equal to about 13 Å for Pr2O3, about 12.4 Å for Nd2O3, about 12.2 Å for Sm2O3, about 13 Å for Gd2O3, and about 13.3 Å for Dy2O3. Consequently, nanolaminates of these lanthanide oxides can be formed with an effective dielectric constants in the range of 11 to 15 and a teq in the range of about 12 Å to about 14 Å.
The formation of the interfacial layer is one factor in determining how thin a layer can be grown. An interfacial layer can be SiO2 for many processes forming a non-SiO2 dielectric on a silicon substrate. However, advantageously, in an embodiment forming a lanthanide oxide nanolaminate with an initial layer of Pr2O3, a thin amorphous interfacial layer is formed that is not a SiO2 layer. Typically, this interfacial layer is either an amorphous layer primarily of Pr2O3 formed between the silicon substrate and a crystalline form of Pr2O3, or a layer of Pr—Si—O silicate. The dielectric constant for Pr—Si—O silicate is significantly greater than SiO2, but not as high as Pr2O3.
Another factor setting a lower limit for the scaling of a dielectric layer is the number of monolayers of the dielectric structure necessary to develop a full band gap such that good insulation is maintained between an underlying silicon layer and an overlying conductive layer on the dielectric layer or film. This requirement is necessary to avoid possible short circuit effects between the underlying silicon layer and the overlying conductive layer used. In one embodiment, for a 0.5 nm interfacial layer and several monolayers of lanthanide grown, an expected lower limit for the physical thickness of a dielectric layer grown by forming a lanthanide oxide nanolaminate is anticipated to be in about the 2-4 nm range. Consequently, typical dielectric layers or films can be grown by forming lanthanide oxide nanolaminates having physical thickness in the range of 4 to 10 nm. The number of layers used, the thickness of each layer, and the lanthanide oxide used for each layer can be engineered to provide the desired electrical characteristics. Pr2O3 used as the initial layer is expected to provide excellent overall results with respect to reliability, current leakage, and ultra-thin teq.
Alternate embodiments include forming lanthanide oxide nanolaminates by electron beam evaporation with target material to form Pr2
other than Pr6
, forming lanthanide oxide nanolaminates by atomic layer deposition, and electron beam evaporation forming lanthanide oxide nanolaminates with initial layers of a lanthanide oxide other than Pr2
. The physical thicknesses can range from about 2 nm to about 10 nm with typical thickness ranging from about 4 nm to about 10 nm. Such layers have an effective dielectric constant ranging from 11 to 31, where a layer with a typical interfacial layer has an effective dielectric constant in the range 11 to 16 and a layer with a significantly thin interfacial layer can attain an effective dielectric constant in the range 25 to 31. Consequently, a range for the equivalent oxide thickness of a dielectric layer formed as a lanthanide oxide nanolaminate can be engineered over a significant range. The expected teq
ranges for various effective dielectric constants are shown in the following
| || |
| || |
| || ||Physical ||Physical ||Physical ||Physical |
| || ||Thickness ||Thickness ||Thickness ||Thickness |
| || ||t = 0.5 nm ||t = 1.0 nm ||t = 2.0 nm ||t = 10 nm |
| || ||(5 Å) ||(10 Å) ||(20 Å) ||(100 Å) |
| ||κ ||teq (Å) ||teq (Å) ||teq (Å) ||teq (Å) |
| || |
| ||11 ||1.77 ||3.55 ||7.09 ||35.45 |
| ||12 ||1.63 ||3.25 ||6.50 ||32.50 |
| ||13 ||1.50 ||3.00 ||6.00 ||30.00 |
| ||14 ||1.39 ||2.79 ||5.57 ||27.86 |
| ||15 ||1.30 ||2.60 ||5.20 ||26.00 |
| ||16 ||1.22 ||2.44 ||4.88 ||24.38 |
| ||20 ||0.98 ||1.95 ||3.90 ||19.50 |
| ||25 ||0.78 ||1.56 ||3.12 ||15.60 |
| ||31 ||0.63 ||1.26 ||2.52 ||12.58 |
| || |
As noted previously, various embodiments provide a typical teq
of about 14 Å. With careful preparation and engineering of the lanthanide oxide nanolaminate limiting the size of interfacial regions, a teq
down to 2.5 Å or lower is anticipated.
The novel process described above provides significant advantages by providing a straight forward method of forming dielectric layers having ultra-thin equivalent oxide thicknesses by electron beam evaporation. Praseodymium oxide-based nanolaminates provide excellent reliability characteristics, based on measurements of current density as a function of gate voltage and stress induced leakage currents for the lanthanide oxides which form the nanolaminates. The dielectric breakdown occurs a least above 43 MEV/cm. The lanthanide oxides forming the nanolaminates retain excellent J-V characteristics even after stress-induced electrical breakdown. Though the praseodymium oxide forming the first layer of the nanolaminate is epitaxially oriented with respect to a substrate on which it is formed, the praseodymium layer is separated from the substrate surface by a thin amorphous layer. Additionally, the novel process and novel dielectric layer structure can be implemented to form transistors, memory devices, and electronic systems including information handling devices.
A transistor 100 as depicted in FIG. 1 can be formed by forming a source/drain region 120 and another source/drain region 130 in a silicon based substrate 110 where the two source/drain regions 120, 130 are separated by a body region 132. The body region 132 separated by the source/drain 120 and the source/drain 130 defines a channel having a channel length 134. Pr2O3 is formed on the body region 132 by evaporation using a electron gun at a controlled rate. Subsequently, another lanthanide oxide selected from the group consisting of Nd2O3, Sm2O3, Gd2O3, and Dy2O3 is formed on the Pr2O3 layer by controlling a rate of electron beam evaporation. This controlled process forms a film 140 containing a nanolaminate of Pr2O3 and another lanthanide oxide on the body region 132. A gate 150 is formed over the gate dielectric 140. Typically, forming the gate 150 includes forming a polysilicon layer, though a metal gate can be formed in an alternative process. Forming the substrate, source/region regions, and the gate is performed using standard processes known to those skilled in the art. Additionally, the sequencing of the various elements of the process for forming a transistor is conducted with standard fabrication processes, also as known to those skilled in the art.
The method of forming lanthanide oxide nanolaminates for a gate dielectric is applied to other transistor structures having dielectric layers in various embodiments according to the teachings of the present invention. For example, the structure of FIG. 3 depicts a transistor 300 having a silicon based substrate 310 with two source/drain regions 320, 330 separated by a body region 332. The body region 332 between the two source/drain regions 320, 330 defines a channel region having a channel length 334. Located above the body region 332 is a stack 355 including a gate dielectric 340, a floating gate 352, a floating gate dielectric 342, and a control gate 350. The gate dielectric 340 can be formed as described above with the remaining elements of the transistor 300 formed using processes known to those skilled in the art. Alternately, both the gate dielectric 340 and the floating gate dielectric 342 can be formed by various embodiments in accordance with the present invention as described above.
Transistors created by the methods described above may be implemented into memory devices and electronic systems including information handling devices. Information handling devices having a dielectric layer containing a lanthanide oxide nanolaminate can be constructed using various embodiments of the methods described above. Such information devices include wireless systems, telecommunication systems, and computers. An embodiment of a computer having a dielectric layer containing a lanthanide oxide nanolaminate is shown in FIGS. 4-6 and described below. While specific types of memory devices and computing devices are shown below, it will be recognized by one skilled in the art that several types of memory devices and information handling devices utilize the invention.
A personal computer, as shown in FIGS. 4 and 5, include a monitor 400, keyboard input 402 and a processing unit 404. The processor unit 404 typically includes microprocessor 506, memory bus circuit 508 having a plurality of memory slots 512(a-n), and other peripheral circuitry 510. Peripheral circuitry 510 permits various peripheral devices 524 to interface processor-memory bus 520 over input/output (I/O) bus 522. The personal computer shown in FIGS. 4 and 5 also includes at least one transistor having a gate dielectric containing a lanthanide oxide nanolaminate in an embodiment according to the teachings of the present invention.
Microprocessor 506 produces control and address signals to control the exchange of data between memory bus circuit 508 and microprocessor 506 and between memory bus circuit 508 and peripheral circuitry 510. This exchange of data is accomplished over high speed memory bus 520 and over high speed I/O bus 522.
Coupled to memory bus 520 are a plurality of memory slots 512(a-n) which receive memory devices well known to those skilled in the art. For example, single in-line memory modules (SIMMs) and dual in-line memory modules (DIMMs) may be used in the implementation of the present invention.
These memory devices can be produced in a variety of designs which provide different methods of reading from and writing to the dynamic memory cells of memory slots 512. One such method is the page mode operation. Page mode operations in a DRAM are defined by the method of accessing a row of a memory cell arrays and randomly accessing different columns of the array. Data stored at the row and column intersection can be read and output while that column is accessed. Page mode DRAMs require access steps which limit the communication speed of memory circuit 508.
An alternate type of device is the extended data output (EDO) memory which allows data stored at a memory array address to be available as output after the addressed column has been closed. This memory can increase some communication speeds by allowing shorter access signals without reducing the time in which memory output data is available on memory bus 520. Other alternative types of devices include SDRAM, DDR SDRAM, SLDRAM and Direct RDRAM as well as others such as SRAM or Flash memories.
FIG. 6 is a block diagram of an illustrative DRAM device 600 compatible with memory slots 512(a-n). The description of DRAM 600 has been simplified for purposes of illustrating a DRAM memory device and is not intended to be a complete description of all the features of a DRAM. Those skilled in the art will recognize that a wide variety of memory devices may be used in the implementation of the present invention. The example of a DRAM memory device shown in FIG. 6 includes at least one transistor having a gate dielectric containing a lanthanide oxide nanolaminate in an embodiment according to the teachings of the present invention.
Control, address and data information provided over memory bus 520 is further represented by individual inputs to DRAM 600, as shown in FIG. 6. These individual representations are illustrated by data lines 602, address lines 604 and various discrete lines directed to control logic 606.
As is well known in the art, DRAM 600 includes memory array 610 which in turn comprises rows and columns of addressable memory cells. Each memory cell in a row is coupled to a common word line. The word line is coupled to gates of individual transistors, where at least one transistor has a gate coupled to a gate dielectric containing a layer Pr2O3 and a layer another lanthanide oxide in accordance with the method and structure previously described above. Additionally, each memory cell in a column is coupled to a common bit line. Each cell in memory array 610 includes a storage capacitor and an access transistor as is conventional in the art.
DRAM 600 interfaces with, for example, microprocessor 606 through address lines 604 and data lines 602. Alternatively, DRAM 600 may interface with a DRAM controller, a micro-controller, a chip set or other electronic system. Microprocessor 506 also provides a number of control signals to DRAM 600, including but not limited to, row and column address strobe signals RAS and CAS, write enable signal WE, an output enable signal OE and other conventional control signals.
Row address buffer 612 and row decoder 614 receive and decode row addresses from row address signals provided on address lines 604 by microprocessor 506. Each unique row address corresponds to a row of cells in memory array 610. Row decoder 614 includes a word line driver, an address decoder tree, and circuitry which translates a given row address received from row address buffers 612 and selectively activates the appropriate word line of memory array 610 via the word line drivers.
Column address buffer 616 and column decoder 618 receive and decode column address signals provided on address lines 604. Column decoder 618 also determines when a column is defective and the address of a replacement column. Column decoder 618 is coupled to sense amplifiers 620. Sense amplifiers 620 are coupled to complementary pairs of bit lines of memory array 610.
Sense amplifiers 620 are coupled to data-in buffer 622 and data-out buffer 624. Data-in buffers 622 and data-out buffers 624 are coupled to data lines 602. During a write operation, data lines 602 provide data to data-in buffer 622. Sense amplifier 620 receives data from data-in buffer 622 and stores the data in memory array 610 as a charge on a capacitor of a cell at an address specified on address lines 604.
During a read operation, DRAM 600 transfers data to microprocessor 506 from memory array 610. Complementary bit lines for the accessed cell are equilibrated during a precharge operation to a reference voltage provided by an equilibration circuit and a reference voltage supply. The charge stored in the accessed cell is then shared with the associated bit lines. A sense amplifier of sense amplifiers 620 detects and amplifies a difference in voltage between the complementary bit lines. The sense amplifier passes the amplified voltage to data-out buffer 624.
Control logic 606 is used to control the many available functions of DRAM 600. In addition, various control circuits and signals not detailed herein initiate and synchronize DRAM 600 operation as known to those skilled in the art. As stated above, the description of DRAM 600 has been simplified for purposes of illustrating the present invention and is not intended to be a complete description of all the features of a DRAM. Those skilled in the art will recognize that a wide variety of memory devices, including but not limited to, SDRAMs, SLDRAMs, RDRAMs and other DRAMs and SRAMs, VRAMs and EEPROMs, may be used in the implementation of embodiments of the present invention. The DRAM implementation described herein is illustrative only and not intended to be exclusive or limiting.
A gate dielectric containing a layer of Pr2O3 and a layer of another lanthanide oxide, and a method of fabricating such a gate dielectric are provided that produces a reliable gate dielectric having an equivalent oxide thickness thinner than attainable using SiO2. Gate dielectric structures that are formed using the methods described herein include nanolaminates of Pr2O3 and another lanthanide oxide selected from the group consisting of Nd2O3, Sm2O3, Gd2O3, and Dy2O3. These gate dielectric structures are thermodynamically stable such that the gate dielectrics formed will have minimal reactions with a silicon substrate or other structures during processing.
Transistors, higher level ICs, devices, and electronic systems are provided utilizing the novel gate dielectric and process of formation. Gate dielectric layers of lanthanide oxide nanolaminates are formed having a high dielectric constant (κ), where the gate dielectrics are capable of a teq of 14 Å or thinner, providing suitable substitutes for SiO2 gate dielectrics. At the same time, the physical thickness of the Pr2O3 layer is much larger than the SiO2 thickness associated with the teq limit of SiO2. Forming the larger thickness provides advantages in processing the gate dielectric. In addition forming a dielectric layer or film containing a lanthanide oxide nanolaminate allows the engineering or selection of a dielectric constant ranging from that of Pr2O3 to a dielectric constant of another lanthanide oxide that is comprised in the nanolaminate.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive. Combinations of the above embodiments, and other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention includes any other applications in which the above structures and fabrication methods are used. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.