The invention relates generally to a nanolaminate structure involving Al2O3 thin films as a main component. More particularly, the invention relates to a nanolaminate structure and method of manufacture of an Al2O3/TiO2 layered structure with selected placement of an Al2O3, layer and in some cases other oxides layers, at the interface of the Al2O3-based nanolaminate structure with a metallic top and/or bottom contact layer to provide a capacitor-like structure.
Extensive basic and applied research is currently being performed in several industrial, national, and university laboratories around the world to develop next-generation low voltage, low energy consumption, high efficiency integrated circuits for advanced microelectronic devices. The heart of every integrated circuit, thus microelectronic devices, is the transistor. The most used transistor scheme in current microelectronic devices is the metal-oxide-semiconductor field-effect-transistor (MOSFET).
For decades, the key element enabling the scaling of the Si-based MOSFET has been the material properties (and resultant electrical properties) associated with the amorphous SiO2 layer used as a gate oxide. The use of the amorphous SiO2 gate oxide layer offered key advantages in the processing and operation of complementary metal-oxide-semiconductor (CMOS) devices, including a stable high-quality Si—SiO2 interface as well as superior electrical isolation properties. Until recently, CMOS devices were based on SiO2 gates that exhibit defect charge densities of ˜1010/cm2 and mid-bandgap interface state densities of ˜1010/cm2. Despite the attributes of SiO2, CMOS devices with gate width ≦45 nm required a 1 nm thick SiO2 layer. The problem with such an extremely thin SiO2 gate layer was that the electron moving from the source to the drain of the CMOS device were leaked to the gate due to quantum tunneling across the extremely thin SiO2 gate. Therefore, the SiO2 gate layer needed to be replaced by a high-dielectric constant (k) layer that could be as thick as 5-10 nm and still keep a high capacitance for a shorter gate because of the higher k value of the dielectric layer, based on the formula for capacitance C=k·A/t, where A and t are the area and thickness of the gate layer, respectively.
For a metal-oxide layer on a p-type semiconductor, when there are no surface electronic states at the oxide-semiconductor interface (see
Increasing VG to a threshold VT that produces enough band bending in the depletion layer results in the appearance of enough free electrons at the semiconductor-oxide interface. The voltage applied between the source and the drain then sweeps the electrons generated in the n-channel, establishing a current that corresponds to the transistor state “on”. If we make the assumption that all the gate voltage above VT is used to accumulate electrons in the depletion layer without increasing its thickness, we can determine the number of free electrons based on the concept of charge on the plates of a capacitor. The value of the capacitance per unit area corresponding to the oxide layer is:
C
ox
=k
ox
/t, (1)
Where t is the oxide thickness and kox is the dielectric constant of SiO2. From the general relation of a capacitor VC=Q, we can calculate Qn, the charge density per unit area of electrons as:
Q
n
=C
ox(VG−VT), (2)
For a gate voltage VG<VT there is no n-channel connection between the source and drain (i.e., the source-drain current Isd=0 irrespective of the drain voltage VD (this is not exactly true because there is always a small leakage current across the reverse bias p-n junction). For VG>VT and VD<VT the n-type channel between the source and drain behaves like a resistor, for which the resistance R is given by:
R=−L/W μ
n
Q
n
e, (3)
where L and W are the length and width of the channel, respectively. The conductance gd of the channel is given by:
g
d
=dI
D
/dV
D(for VG=constant), (4)
and for the linear regime for which ID=VD/R, gd is given by:
g
d=1/R=eμnQnW/L, (5)
where μn is the mobility of the electrons in the channel between the source and the drain.
The actual physical thickness of a high-k layer needed to achieve the equivalent capacitance of a 1 nm thick SiO2 layer is give by the following equation:
t
high-k
=k
high-k
·t
eq/3.9, (6)
so a dielectric layer with a dielectric constant k=16 results in a required physical thickness of ˜4 nm to obtain a teq=1 nm. Therefore, the higher the dielectric constant k the thicker the gate dielectric layer can be, while still possessing the equivalent oxide thickness of a 1 nm SiO2 layer.
The HfO2 layer currently used in the first generation of commercial CMOS devices with gate width ˜45 nm exhibits a dielectric constant k=20. However, for the next generation of CMOS devices with shorter gates, there will be a need for materials with dielectric constant k≧30. A TiAlOx alloy has been developed with k=30 and low leakage, providing an alternative gate oxide for nanoscale CMOS devices. However, the TiAlOx alloy exhibits a bandgap of about 4 eV, which is still below the 5.5 eV bandgap needed for reliable operation of a high-k dielectric gate layer.
Even, more recently, it has been demonstrated that Al2O3/TiO2 nanolaminates exhibit dielectric constants of up to k=1000. However, the capacitive structures fabricated with those Al2O3/TiO2 nanolaminates on Si substrates exhibit relatively high leakage, and the high dielectric constant of ˜1000 decreased rapidly below about 100 at about 1 kHz, giving only a limited frequency range of operation. The limited operation of the Al2O3/TiO2 nanolaminates without the interfacial Al2O3 layer at the top electrode/nanolaminate interface is due to the fact that there is a substantial charge injection into the nanolaminates.
In early 2007, INTEL announced the deployment of HfO2-based high-k dielectrics in conjunction with a metallic gate for components built on 45 nm CMOS technologies, and shipped it in the 2007 processor series codenamed Penryn. However, although the first generation of nanoscale CMOS devices with high-k dielectric gates are already in the market, the dielectric constant of HfO2 and most amorphous oxide dielectric known today is much less than 100.
Another critical technological application of the Al2O3/TiO2 nanolaminates can be a new generation of supercapacitors for energy storage in industrial applications and for biocompatible (TiO2 and Al2O3) energy storage capacitors embedded in microchips implantable in humans as part of biomedical devices for restoring sight to blind people, or restoring hearing to deaf people, or restoring other degraded human functions.
A nanolaminate structure is made of Al2O3/TiO2 multilayered thin films and an oxide layer preferably at the interface between the top electrode and the nanolaminate. The nanolaminate thin films have individual layer thicknesses in the range of about 0.1 to 1 nm and beyond, as required to achieve the combined properties of giant dielectric constant (k=100-1000 or higher), low leakage current, and low losses for extended frequency range operation. The combined properties mentioned above are achieved by inserting an Al2O3, or in selected cases can be achieved by inserting another oxide (e.g., HfOx, BaOx, TaOx, NbOx, etc.) at the interface of the Al2O3/TiO2 nanolaminate with a metallic (e.g., Pt, Ti, W, or any other metal) bottom and/or top contact layers to produce capacitor-like structures. Such structures can be used in several new technologies for which the giant-k dielectric Al2O3/TiO2 nanolaminate provides a critical component, namely: 1) as a gate oxide for the new generation of nanoscale CMOS transistors for integrated circuits in nanoelectronics; 2) as a dielectric layer for fabrication of high-capacitance capacitors embedded in microchips implantable in the human body (using the biocompatible properties of Al2O3 and TiO2 films); 3) as a dielectric layer for fabrication of high-capacitance capacitors for energy storage systems; 4) as a nanoscale insulator layer for magnetic multilayer memory devices; and 5) for any other application where an atomic scale thick layer with high-dielectric constant is required. A great advantage of the Al2O3/TiO2 nanolaminates is that the individual layers are amorphous, thus being much simpler and less costly to produce on different substrates used for the fabrication of devices. The Al2O3/TiO2 nanolaminates with the interfacial Al2O3 layer at the nanolaminate/metallic contact layer solve the problem of the high leakage/losses and limited frequency range operation of the nanolaminates without the interfacial Al2O3 layer, thus making the nanolaminates suitable for commercial devices.
These and other advantages and features of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.
a) shows a schematic of a metal-insulator capacitor structure including a TAO (Al2O3/TiO2) nanolaminate structure as a dielectric layer;
a) shows a high resolution cross-section TEM image of the TAO nanolaminate with 0.5 nm thick individual layers of Al2O3 and TiO2;
a) shows dielectric constant versus frequency;
a) shows a schematic of various positions of additional Al2O3 layer to the TAO nanolaminate structure;
a), 2(b), 3(a) and 3(b) show a nanolaminate structure 100 having a plurality of Al2O3/TiO2 thin films 110/120 having layer thicknesses of about 0.1-1 nm, although greater nm thicknesses can also be used, provided combined properties of the nanolaminate structure 100 achieve a giant dielectric constant of k−100-1000 or higher, low leakage currents and low losses for extended frequency range operation.
In addition to the application for the next generation of nanoscale CMOS devices, the new Al2O3/TiO2 nanolaminates 110/120 can include an interfacial Al2O3 layer 140 to provide a reliable dielectric layer for microchip embedded capacitors for implantable biomedical devices, and for high capacitance capacitor for energy storage systems, and for other future applications requiring high-k dielectric layers with all the properties shown for the new nanolaminates reported here.
In one preferred embodiment shown in
The Al2O3/TiO2 nanolaminate structure 100 is preferably grown using atomic layer deposition (ALD), which is the technique capable of growing films with atomic layer precision. All layers were grown at 300° C., although lower temperatures (100-200° C.) can also produce the Al2O3/TiO2 nanolaminates. The ALD process uses trimethylaluminum [Al(CH3)3] and titanium tetrachloride (TiCl4) as Al and Ti precursors, respectively, and H2O as the oxidant.
For the preferred Al2O3/TiO2 nanolaminate structure 100, 5 cycles of Al2O3 and 15 cycles of TiO2 sublayers were deposited alternately to achieve equal sublayer thicknesses of 0.5 nm. A reference Al2O3/TiO2 nanolaminate was used to precisely elucidate the effect of the Al2O3 interfacial layer the between Al2O3/TiO2 nanolaminate structure 100 and the top electrode contact 150. Nanolaminates with additional layers associated with the top electrode contact 150, a middle layer 170, and a bottom Al2O3 interfacial electrode contact 180 can be referred to as A-TAO, TAO-A-TAO, and TAO-A, respectively. Therefore, for example, 4.5A-TAO designates that a 4.5 nm thick Al2O3 interfacial layer 140 is deposited adjacent the top electrode contact 150 of the reference TAO nanolaminate structure 100. A Pt form of the top electrode contact 150 and the bottom electrode contact 180 can be grown by RF-magnetron sputtering, electron beam evaporation, or atomic layer deposition for both the bottom and top electrode contact 150, using a shadow mask to define a circular or any other geometrical shape of the top Pt electrodes 150 and 180; thickness and diameter (defined by a shadow mask) of the top electrode contact 150 was 100 nm and 250 mm, respectively. Alternatively, the patterned top electrodes can be produced by growing a blanket metal film on the nanolaminate and then do lithography and reactive ion etching (RIE) to define the geometry and dimensions of the electrodes. X-ray diffraction (XRD, Philips X-Pert 8856) and Transmission Electron Microscopy (TEM, FEI Tecnai F20ST TEM/STEM) revealed that the Al2O3 and TiO2 layers of the nanolaminate structure 100 are amorphous. Dielectric properties were measured using an Agilent 4294A precision impedance analyzer at room temperature. The leakage current was measured using a Keithley 237 unit.
a) illustrates a representative Pt/TAO/Pt capacitor nanolaminate structure 220 on a TiO2-coated Si substrates 230. A reference TAO nanolaminate structure 100 was terminated by 0.5 nm thick top and bottom Al2O3 interfacial layers 140 with a symmetric layered structure, as depicted in
a) shows a high resolution cross-section TEM image of the TAO nanolaminate structure 220 with 0.5 nm thick individual layers of the Al2O3 layer 110 and the TiO2 layer 120;
The effects of top Al2O3 interfacial layers 140 on dielectric constant, loss, and leakage current density of the TAO nanolaminates 100 are shown in
It was observed that a 1.5 nm Al2O3 interfacial layer 140 is too thin to block leakage current between a top electrode contact 150 and the dielectric interfacial film layer 140. Such behavior is likely related to conduction mechanisms of ALD Al2O3 ultrathin films reported in the art (but such suppositions are not meant to limit the claims) wherein direct tunneling occurred when ALD Al2O3 ultrathin interfacial films 140 are thinner than 2.5 nm. It is also reported that the dielectric strength of the ALD Al2O3 ultrathin films 140 could reach ˜10 MV/cm when deposited on very smooth conductive substrates. In the case, of the dielectric interfacial films 140 discussed herein, since the substrate has been coated with sputtered polycrystalline Pt films, thus exhibiting surface roughness larger than the conventional atomic scale roughness of a Si surface, the thickness of the ALD Al2O3 film 140 that can effectively reduce tunneling seems to be 3.5 nm. This is probably due to high electric fields developed on nanoscale asperities resulting from the sputter-deposition process of Pt films. To further reduce the tunneling current (<10−7 A/cm2 at 1 V), it is necessary to increase the thickness of the Al2O3 interfacial layers 140 approximately to >4.5 nm.
The following non-limiting examples illustrate various aspects of the invention.
In order to optimize the effect of the Al2O3 interfacial layer, experiments were performed to determine the optimum position of the Al2O3 interfacial layer, considering effects of surface roughness or sputter-deposition-induced topography during the electrode layer growth. In this respect, studies focused on determining the effect of inserting the 4.5 nm thick Al2O3 interfacial layer at different positions in the TAO nanolaminates, as schematically depicted in
The capacitor structures produced with the top Pt electrode deposited by e-beam evaporation result in substantial less leakage current density (not shown here) than that of the electrode produced by sputter-deposition. On the other hand, for the case of insertion of the Al2O3 interfacial layer at the bottom electrode (TAO-4.5A), it does not make a big difference because the interface is very rough as seen in
The present invention has the potential of providing a whole new generation of nanoelectronic devices, which require high-k dielectric layers with the combined properties demonstrated for the Al2O3/TiO2 nanolaminates. The high dielectric constant TAO nanolaminates also provide the basis for new embedded supercapacitors in a new generation of microchips implantable in the human body, due to the biocompatibility of TiO2 and Al2O3. Also, the TAO-based supercapacitors can provide a new generation of energy storage systems. Because the component layers are biocompatible, these nanolaminates provide a broad range of applications to both non-biologically and biologically compatible devices and systems.
The foregoing description of embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments, and with various modifications, as are suited to the particular use contemplated.
The United States Government claims certain rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the United States Government and the University of Chicago and/or pursuant to DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory.