This invention relates to a new electronic memory mechanism involving the movement of protons (hereinafter hydrogenous ions) within a dielectric layer. The memory mechanism may manifest as a static random access memory, a dynamic random access memory, a nonvolatile memory, an optical memory, or as a flat panel display. The memory elements are most readily constructed in silicon-based structures.

There is a wide variety of structures that can act as memory elements. Each has its own advantages and disadvantages. The ubiquitous dynamic random access memories (DRAM) have high capacities, relatively low cost, and fast access times but are subject to upsets from radiation and other mechanisms and require refresh and rewrite circuits.

Static RAMs require more circuitry and also are subject to upsets. Flash and silicon- nitride based (SONOS) nonvolatile memories have good access times and endurance but require high (12-15V) programming voltages. EEPROM nonvolatile memories have similar characteristics. Ferroelectric memories are incompatible with silicon processing but are otherwise attractive nonvolatile memories. What is needed is a low cost, silicon compatible, low voltage write/read memory that is radiation tolerant and nonvolatile with reasonable write speeds and endurance. Unfortunately no such device exists in the art at this time.

Summary of the Invention The basic mechanism exploited herein to form the memory elements of this invention is the movement of ions containing a single proton (hydrogen+, deuterium+, and tritium+--hereinafter hydrogenous ions) within a dielectric layer contained between two semiconducting or conducting layers that are impermeable to the ions. An electric field is applied reversibly across the dielectric layer to change the spatial position of the hydrogenous ions within the dielectric layer. The position of the hydrogenous ions remains the same and endures unless intentionally moved by the application of a reversed electric field across the dielectric layer. Since the protons create their own positive electric field, their spatial position within the dielectric layer can be used to form a memory element.

When implemented as a microelectronic memory, silicon dioxide is a preferred material for the dielectric layer. The hydrogenous ions are introduced into the dielectric layer either by a high temperature anneal in a hydrogenous gas atmosphere or by implanting hydrogenous ions directly into the dielectric layer. The layers above and below the dielectric layer comprise a material that is highly imperious to the migration of the hydrogenous ions with poly or single crystalline silicon as a preferred choice.

Electrodes are associated with the layers above and below the dielectric layer to create the electric field to change the spatial position of the hydrogenous ions. The state of the memory element is determined by sensing the conductivity of either of the layers adjacent to the dielectric. If the hydrogenous ions have been moved next to the interface between the dielectric layer and the adjacent semiconducting layer, the positive electric field from the ions will attract electrons from the adjacent semiconducting layer to the interface thereby changing the conductivity. When realized with silicon dioxide as the dielectric layer, the memory element is nonvolatile. When other dielectric materials are employed, the memory element can be a nonvolatile memory or a DRAM that is radiation tolerant When implemented as an optical memory element, an additional component is added. This is an electro-sensitive optical layer that is disposed between the upper layer and the top electrode which is now made of a transparent electrode material. The optical layer is sensitive to the spatial position of the ions and their associated electric field. An optical characteristic of the layer changes in response to the presence or absence of this field. The element would typically be formed as part of a large array of elements, each of which could be individually read by light passing completely through the element or reflecting off the side with the optical layer. The array of elements could also be configured as a flat panel display.

Brief Description of the Drawings Figure 1 A is a cross sectional view of one embodiment of the electronic memory element formed as a pseudo (T) MOSFET with the hydrogenous ions at the upper boundary of the oxide layer, and Figure 1B is a simplified graph of the current voltage (I-V) characteristic of the element.

Figure 2A is a cross sectional view of the embodiment of the electronic memory element shown in Fig. lA with the hydrogenous ions at the lower boundary of the oxide layer, and Figure 2B is a simplified graph of the I-V characteristic of the element.

Figure 3A is a graph of the I-V characteristic of a SIMOX T-MOSFET structure, and Figure 3B is a graph of the C-V characteristic of an MOS capacitor, as shown in the respective inserts.

Figures 4A and 4B show, respectively, drain current decay and a cross sectional schematic representation of the migration of the hydrogenous ions causing the decay.

Figure 5 is a graph showing initial rate of decay of drain current as a function of gate bias. Each data point represents the initial slope of the 1-t curves of Fig. 4A.

Different 1-t curves were obtained by first applying a + 60 V substrate bias for 5 minutes to accumulate the ions at the interface between the oxide layer and the upper Si layer, followed by switching to different negative substrate biases. The dashed line is a y = ax2 least squares fit to the data (a = 0.002).

Figure 6A is an Arrhenius plot of the initial rate of decay of the normalized drain current. The data were taken as in Fig. 5 by first applying a + 60 V substrate bias for 5 minutes, but now followed by switching to-10 V substrate bias at different substrate temperatures. Figure 6B shows the normalized decay of the drain current vs. time at 73° C.

Figure 7 is a cross sectional view of another embodiment of the electronic memory element, here an embedded memory element for use in conjunction with non- memory elements on the same silicon substrate.

Figure 8 is a graph of the measured I-V characteristic for the element of Fig. 7.

Figure 9 is a graph showing the retention characteristic of the element of Fig. 7 at room temperature.

Figure 10 is a graph showing the retention characteristic of the element of Fig. 7 at high temperature.

Figures 11A and 11B are endurance and radiation tolerance response graphs for a memory element of this invention and a conventional electronic memory element, respectively.

Figure 12 is a cross sectional view of an optical memory element used reflectively.

Figure 13 is a cross sectional view of an optical memory element used transmissively.

Detailed Description of the Invention The effects of hydrogen ions in microelectronic devices have been studied for some time. Metal-oxide-semiconductor (MOS) devices when exposed to ionizing radiation or hot carrier stress degrade due to the creation of electrically active defects at the oxide-semiconductor interface. Many studies suggest that ionic hydrogen plays a key role in this degradation. In the course of studying these problems, the important discovery of the hystersis of the current-voltage (IV) characteristics in a dielectric layer containing such protons and surrounded by conducting or semiconducting layers was made by the inventors herein. The initial discovery was made in the course of investigating the silicon-on-insulator (SOI) material system but has since been expanded to more conventional silicon processing systems.

As was mentioned above, the fundamental proton displacement mechanism is applicable to electronic memory and also to optical memory that is optically read but electrically written. The electronic memory embodiment will be discussed first.

Electronic Memory The mechanism embodied in an electronic memory is perhaps most readily understood by study of Figures lA, 1B, 2A and 2B. Figure lA is a simple embodiment of the electronic memory with the oxide layer 10 surrounded by the silicon substrate 12 below and the upper p-Si layer 14 above. A gate electrode 18 is attached to the bottom of the substrate, and source 20 and drain 22 electrodes are attached to n+ regions of the upper layer 14. The mobile hydrogenous ions 16 are at the top of the oxide layer. This spatial position of the ions produces the'ON'condition shown in the simplified I-V curve in Figure 1B. The presence of the positively charged ions 16 attracts the mobile electrons in the p region of the upper layer 14 to the interface between layers 10 and 14, thereby creating a conducting channel at the base of the upper layer 14 so that current will easily pass between the source and drain electrodes. Figure 2A shows the same device with the hydrogenous ions 16 now moved to the bottom of the oxide layer 10. In this position the electric field created by the ions is no longer strong enough to create the conductive channel in the upper layer 14. This produces the'OFF'condition shown in the I-V curve in Figure 2B. The ions 16 are moved by placing the voltage biases shown on the gate electrode 18 as shown in Figures 1A and 2A. These devices could also be fabricated with p+ regions at the source and drain electrodes.

The initial devices created to demonstrate the invention were formed in Si/SiO2/Si structures. H+ (D+) ions were introduced into the buried SiO2 layer by annealing the structure in a hydrogen (deuterium) containing atmosphere. The spatial position of the ions in the oxide layer was monitored by high frequency. capacitance voltage and current voltage measurements (Figures 3A and 3B). The reversibility and transient behavior of the proton and deuteron migration inside the oxide layer was studied as a function of gate bias across the oxide at different temperatures and demonstrated that the ions created by the introduction of the hydrogenous gas were responsible for the hysteresis effects observed.

Three different types of Si/SiO,/Si materials were investigated. Separation by the implantation of oxygen (SIMOX) samples were formed by implanting ap-type Si (100) substrate with 1 90-LceV O+ ions to a dose of 1.8 x 10 18 cm-2 followed by a subsequent anneal at 1320 °C in Ar + 1% 02. This resulted in a 200-nm monocrystalline Si (c-Si) layer on top of a 400-nm buried oxide layer, so the oxide layer is sandwiched between two c-Si layers. Zone-melt-recrystallization (ZMR) SOI wafers, p-type (100) Si material.- with a 330-nm monocrystalline Si layer on top of a 1-um-thick buried oxide layer, were also investigated. Apart from these two SOI materials we also studied a standard thermal Si02 (40-nm thick) capped with an undoped poly-Si layer (1-um thick), chemical-vapor- deposited and annealed at 1200 °C for 2 h in Ar + 1% 02.

Part of the top Si layer of the wafers was etched in a HN03-CH3COOH-HF mixture through a mask, leaving behind isolated rectangular strips (2 mm by 9 mm) of top Si layer. This was done to reduce leakage currents at the substrate edges and through the buried oxide layer during the subsequent point contact pseudo-metal-oxide- semiconductor-field effect-transistor (T-MOSFET) measurements, and to facilitate lateral diffusion of hydrogen (deuterium) into the buried oxide during the forming gas anneal.

This lateral diffusion is crucial because the amount of hydrogen diffusing through the top c-Si layer into the buried oxide is negligible, due to the very low solubility of hydrogen in si.

Forming-gas (FG) [N2: H2; 95: 5 or N2 : D2; 95: 5 (by volume, 99.999% pure)] and nitrogen (99.999% pure) anneal treatments were performed using a flow through a quartz tube inserted into a tube furnace between 300 and 900 °C for 30 min. For the capacitance-voltage (C-V) measurements, the Si strips were etched off using KOH. C-V measurements on the Si/Si02 structures resulting from the KOH etch were made at 1 MHz with a mercury probe or Al dots (~ 1 mm2 contact area) forming gate contacts.

Current-voltage (1-T/) measurements on the Si/Si02/Si structures were performed using the point-contact T-MOSFET technique. The buried oxide plays the role of the gate dielectric and the top Si layer represents the transistor body. Two tips of a standard four point probe are placed on the top Si layer to form the source and drain point contacts, while the gate voltage is applied to the back of the Si substrate.

Figure 3A shows the hysteretic behavior of the I-Vcurves on SIMOX after it received a 550 °C FG anneal. Similar features were observed in the ZMR and the poly-Si capped thermal oxide, and occur over a FG-anneal temperature range from 500 to 800 °C.

Curve 1 was recorded with a decreasing gate (substrate) bias (from positive to negative) after the bias was kept constant at the initial value (+ 40 V) for 5 minutes. Curve 2 was subsequently recorded using the opposite gate voltage sweep direction (from negative to positive) after holding the gate bias at the initial value (-70 V) for 5 min. The "hysteretic"behavior is evidenced by the fact that the original I-V curve (curve 1) was not retraced by reversing the voltage sweep direction (curve 2). Curve 3 was recorded after curve 2 using the same procedure described for curve 1, showing the reversibility of the process.

In general, charge in the oxide will cause the I-V curve to shift along the voltage axis. This voltage shift (AP) is proportional to the charge density and depends on its spatial distribution in the Si02 layer: where e is the electronic charge, d is the oxide thickness, s is the oxide dielectric constant, and p (x) is the local charge density in the dielectric. For the MOS capacitors. the variable"x"is the distance between the local oxide charge r (x) and the metal gate ; for the T-MOSFET, x is the distance between the charge and the substrate-Si/SiO interface.

As can be seen from the integral in Eq. 1, AV is maximized for the T-MOSFET if the charge is located near the top-Si/Si02 interface (x~ d) and is minimized if it is located near the substrate-Si/Si02 interface (x ~ 0). The negative voltage shifts AV in the I-V plots are caused by positive charges in the buried Si02 (areal density 2 x 1012 cmr).

The observed hysteretic behavior is the result of an electric field induced migration of a charged ionic species from one Si/SiO2 interface to the other. This type of behavior was not observed after annealing in Ar or N2, showing that the presence of hydrogen in the anneal ambient triggers the hysteretic behavior; it is not due to mobile ion contamination.

If the top Si layer is removed and an MOS capacitor is formed, it is observed that the charged species escape. the Si02 dielectric through the metal gate under negative gate bias. This further demonstrates that the mobile species involved are positively charged.

The crucial role of the hydrogen anneal step to trigger the hysteretic effect, and the much higher solubility of hydrogen and reaction in metal capacitor gates such as Al or Hg as compared to Si, collectively suggest that the mobile charge is H+.

The transient behavior of the field induced charge migration from an accumulated interfacial layer into the bulk of the buried Si02 layer was analyzed as a function of different gate voltages (VG) and temperatures. This was done by first applying a positive VG to the substrate of Si/Si02/Si device structures, which causes positive ion accumulation in a thin layer at the top-Si/SiO2 interface (areal density QH+). After this, the polarity of VG was reversed, forcing the ions to migrate away from the accumulated layer into the bulk of the Si02. The exodus of ions away from the charge layer accumulated at the top-Si/Si02 interface, induced by the applied VG, is well described as space-charge-limited current. Space charge limited current flows when mobile charge carriers are injected into an insulator, where no other charges are present. Solving Poisson's equation for this problem yields: DOH where Dt is the rate of decrease in the charge density accumulated at the top-Si/SiO2 interface, JH is the space charge limited ionic current density in the SiO layer. and H+ is the ionic mobility.

When the positive ionic charge, QH+, is accumulated at the top-Si/SiOn interface in a T-MOSFET structure it will induce a negative threshold voltage shift, A V, in the transistor characteristics. As a result, an inversion channel is formed in the p-type top Si layer at the top-Si/SiO ? interface, and a drain current (ID) will flow in the T-MOSFET device. We will assume a linear relationship between OH+ and the resulting threshold voltage shift A Vat the top-Si/buried-SiO2 interface at any time during the field induced ionic drift. This is only an approximation since the exact expression for the threshold voltage shift involves integration over the entire oxide layer as described in Eq. 1. We will also assume a linear relationship between A V and the T-MOSFET drain current at any gate voltage VG smaller than the voltage shift A V. As can be seen from Fig 3A this appears to be a reasonable approximation. Consequently, the decrease in QH+ as a A0.- function of time after switching the gate bias polarity, #, can be measured as a <BR> <BR> <BR> <BR> #ID<BR> <BR> <BR> decrease in ID over time (#). Figure 2 (a) shows the decay of ID vs. time in a<BR> #t<BR> <BR> <BR> <BR> <BR> <BR> T-MOSFET device on FG-annealed ZMR material. The positive ions were first drawn to the top-Si-Si interface with a VG = + 60 V substrate bias, followed by switching to VG =-10 V and maintaining the substrate at 73 °°C. The schematic in Fig. 4B shows an SOI cross section, with the evolution of the conduction electrons in the top-Si inversion layer and the ionic distribution, on the same time scale as Fig 4A. The initial rate of #ID decay of the #-MOSET drain current # obtained from the ID vs. t curves such as #t the one shown in Fig. 4A was also analyzed. The VG dependence was measured by switching to different negative VG after first accumulating the ions at the top-Si/Si02 interface. It was found that the electric field dependence of the initial decay rate is well described by a square-law behavior as anticipated from Eq. 2. It should be noted that the relatively high voltage employed and the relatively long time scale are a function of the relatively thick oxide layer tested here.

To first order, the ionic mobility in Eq. 2 is governed by a thermally activated hopping process and is given by: o<BR> I1 6kBT P (kBT)) (3) where q is the ionic charge, a is the intersite hopping distance, Eu is the activation energy, kB is.-Boltzmann's constant and T is the absolute temperature. The parameter is the attempt to escape frequency, in which 0"is the second derivative of the function describing the potential-well of the hopping site, and m is the mass of the ionic species. Hopping simply implies that the ion moves through the lattice by consecutive jumps (hops) from one site to another, and that the ion must have enough energy (> Ea) before the jump can occur with any reasonable probability.

To determine Ea for H+ motion, the initial rate of decay of the #-MOSFET drain #ID current # in FG-annealed ZMR devices was again measured in the same way as #t described above, except that the temperature was varied at a fixed VG (-10 V). Figure 5 zip T shows the Arrhenius plot of---vs. 7 (see Eqs. 2 and 3) measured on Si/SiO2/Si (ZMR) structures that were hydrogenated or deuterated at 600° C. From this plot, we deduce an activation energy Ea # 0.8 eV for the ionic mobility for both the hydrogenated and deuterated buried SiO2. This value is in good agreement with the diffusion activation energy of H+ in standard thermal Si02 thin films. The difference in absolute values between the two curves in Fig. 6A and 6B results from a difference in the prefactor of Eq. 3 of 1.5, again in good agreement with the theoretically expected difference of This observed hysteretic behavior can be utilized in a non-volatile NVFET memory device. An n-channel transistor can be changed to"normally on"or"normally off'by applying a positive or negative gate (substrate) bias which will drift the protons to the top Si/SiOw or substrate Si/SiO interface, respectively. For a memory device this can be interpreted as writing the device to a bit state"I"or"0", respectively. To read the device, the zero bias drain current/0 is simply measured (high current then corresponds to logic state"I", low current to"0"), as visualized in Figs. 1, 2, 3 and 4. In contrast with the instabilities that are usually associated with mobile ions in Si02, charge retention experiments on this device show that once written to a specific state, the device remains in that state for over 104 s while heated to 200° C. Furthermore, fatigue experiments performed on these structures show that the device easily endures over 104 write-erase cycles without any degradation.

For memory devices, a short write time is desirable. For the device used in Fig. 4 the'write time would be about 100 s. It follows from Eq. 2 that the device speed is proportional to d-3, where d is the buried oxide thickness. For devices fabricated using the poly-Si-capped 40-nm thermal oxide substrate as shown in Figure 7, a write time of about 30 ms was observed at room temperature, in good agreement with the d-3 dependence predicted from theory. Write times as fast as 1 ms can be expected for 1 0-nm thermal oxides. Even more significantly, the latter result demonstrates that these non- volatile field effect transistor (NVFET) memory devices do not require SOI substrates, but can be fabricated using thermal oxides capped with a poly-Si layer, a standard procedure in Si-MOS processing.

Turning to Figure 7 for more detail on this alternative device, a p-type layer 42 is formed on a silicon substrate 40. An oxide layer 44, which can be either a thermal oxide layer or a deposited oxide layer, is formed on top of the p-type layer 42 and capped by a polycrystalline silicon layer 46. The hydrogenous ions are introduced into the oxide layer at this point, either by annealing in a hydrogenous gas atmosphere or by ion implantation.

The n+ drain 48 and source 50 regions are formed, and finally the various electrodes are formed. To write the device, normally substrate 41, source 51, and drain 49 electrodes would be connected to ground with the gate electrode being biased positively or negatively to change the spatial position of the hydrogenous ions. Figure 8 is a graph showing the I-V characteristic of the device of Figure 7, where the ions were formed in the oxide layer by a 30 minute anneal at 550 ° C in forming gas. This device could also be fabricated with p+ regions about the source and drain electrodes.

This NVFET device has potential advantages over state-of-the-art non-volatile memory technologies such as Flash and EEPROM. While its speed, retention and lifetime performance are certainly competitive with these existing technologies, it is simpler in design, requires fewer processing steps, operates at much lower voltages.

Finally, we discuss the mechanism causing the incorporation of mobile H+ ions in the buried Si02 layer of these Si/Si02/Si materials during the FG anneal. It is known that the high-temperature (1200-1325 °C) formation anneal step creates neutral O vacancies (Si-Si bonds) in the buried oxide via O out-diffusion from the Si02 into the top and substrate Si layers. Because these strained Si-Si bonds can act as H2 cracking sites in the buried Si02, they are catalyst sites for the generation of protons at elevated temperature. Since the solubility of hydrogen species in c-Si is low, once formed, the H+ is largely"imprisoned"in the buried Si02 layer, sandwiched between the two encapsulating Si layers, i. e., the interfaces form a diffusion barrier for hydrogen. While there seems to be a peculiar affinity between Si02 and the hydrogenous ions that provides the excellent characteristics for nonvolatile memory, the scope of the invention is intended to encompass any material which provides similar behavior with hydrogenous ions.

These cracking sites form naturally by the high temperature annealing process as the O from the silicon dioxide structures in the oxide diffuse across to the pure silicon regions in the layers on either side. As the O migrates away, Si-Si bonds are created in the oxide layers that form the cracking sites. However, these Si-Si bonds may also be created deeper within the oxide layer by implanting Si ions into the oxide layer to form the cracking sites. Once the cracking sites have formed, one should avoid further prolonged long duration high temperature processing steps for the devices. Short duration processes such as Al metallizations at 450 ° C or lower are permissible, but higher temperatures should be avoided.

The silicon layers above and below are essentially impermeable to these ions.

However, there may be some devices that require that a layer other than silicon be emplaced next to the central dielectric layer. This material must also act to confine the hydrogenous ions within the dielectric layer. Suitable materials include, but are not limited to, nitrides, nitrided oxide, silicon nitride, and polycrystalline and monocrystalline silicon. Once the structure has been annealed and the ions created, the ions tend to remain within the oxide layer and gaps in the surrounding impervious confining material can be tolerated. Figures 9 and 10 illustrate the excellent retention characteristics of the silicon dioxide devices.

The hydrogenous ions present small capture cross-sections to electrons and, as such, enable these devices to be radiation tolerant. Further experimentation may extend this property to the radiation hard level. Figures 11 A and 11 B compare, respectively, a typical device of this invention with a typical prior art nonvolatile memory on the basis of mobile charge density vs. time of exposure to radiation, demonstrating that the protons have much longer endurance than holes.

The above examples have discussed structures formed by SIMOX processing and conventional silicon MOS processing techniques. Other processes may be employed to form the memory elements including but not limited to zone melt recrystallization (ZMR), UNIBONDO processing, and bonded and etchback silicon on insulator (BESOI).

Optical Memoz l The above described electronic memory elements can also be used to create optical memory elements that are electrically written and erased and optically read. The basic functional element of the dielectric layer containing the mobile hydrogenous ions flanked by layers that confine the ions remains and is combined with the added component of an optically active layer that is sensitive to the proximity of the electric field created by the mobile ions. This optical layer could be a transparent piezoelectric layer whose refractive index changes with the electric field or a liquid crystal layer whose polarization changes with the electric field or other materials with the same or other optical properties affected by the electric field of the mobile ions.

Figure 12 is a view of a reflective embodiment of the optical memory element.

The dielectric layer 50 is surrounded by a back side layer 52, typically a silicon substrate, and by a front capping layer 54, typically a thin silicon layer. The optically active layer 56 located between the front capping layer 54 and a transparent electrode layer 58, preferably tin-oxide or indium tin oxide. The mobile hydrogenous ions are introduced into the dielectric layer as discussed above and prior to formation of the optical layer 56.

Light from an optical source 64 passes through the first polarizer 62, enters the optical layer 56, here a liquid crystal with a polarizing effect upon the light. The light leaves the element, and passes through another polarizer 62 to reach an optical detector. If the mo, bile hydrogenous ions have been moved close to the front capping layer 54, the liquid crystal layer 56 will be in one polarization state; and, if the ions have been moved away from the front capping layer 56, the liquid crystal will be in another polarization state.

The particular polarization state the optical memory element is in will determine whether light transmitted from the source 64 will reach the detector 66. If the optical layer were a piezoelectric material, the polarizers 62 would not be needed. Instead, the refractive index of the optical layer 56 would change and the exit beam would be angularly displaced either onto or away from the fixed detector in this alternate embodiment.

Figure 13 is a view of a transmissive embodiment of the optical memory element.

The differences between this embodiment and the embodiment of Figure 12 are that there is an additional transparent substrate layer 53 typically made of quartz or glass, with a thin silicon layer 55 serving as a rear capping layer to confine the mobile hydrogenous ions on the back side of the dielectric layer 50, here silicon dioxide. As this is a transmissive element, the optical source 64 has been moved to the back side of the element along with one of the polarizers 62. The operation is otherwise similar to that discussed above for the liquid crystal embodiment and for the piezoelectric material embodiment.

If the individual optical memory elements were combined into a large array with individually electrically written elements, the array would become a large scale memory array or, with suitable redesign, the basis for a flat panel display.