US20060024878A1

US20060024878A1 - Deaprom having amorphous silicon carbide gate insulator

Info

Publication number: US20060024878A1
Application number: US11/236,101
Authority: US
Inventors: Leonard Forbes; Joseph Geusic; Kei Ahn
Original assignee: Micron Technology Inc
Current assignee: Micron Technology Inc
Priority date: 1997-07-29
Filing date: 2005-09-27
Publication date: 2006-02-02
Also published as: US7196929B1

Abstract

A floating gate transistor has a reduced barrier energy at an interface with an adjacent amorphous silicon carbide (a-SiC) gate insulator, allowing faster charge transfer across the gate insulator at lower voltages. Data is stored as charge on the floating gate. The data charge retention time on the floating gate is reduced. The data stored on the floating gate is dynamically refreshed. The floating gate transistor provides a dense and planar dynamic electrically alterable and programmable read only memory (DEAPROM) cell adapted for uses such as for a dynamic random access memory (DRAM) or a dynamically refreshed flash EEPROM memory. The floating gate transistor provides a high gain memory cell and low voltage operation.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Ser. No. 09/134,713, filed on Aug. 14, 1998; which is a divisional of U.S. Ser. No. 08/902,843, filed Jul. 29, 1997, now abandoned; each of which is incorporated herein by reference in its entirety.
This application is related to the following co-pending, commonly assigned U.S. patent applications: “MEMORY DEVICE,” Ser. No. 08/902,133; “DEAPROM AND TRANSISTOR WITH GALLIUM NITRIDE OR GALLIUM ALUMIUM NITRIDE GATE,” Ser. No. 08/902,098, now issued as U.S. Pat. No. 6,031,263; “CARBURIZED SILICON GATE INSULATORS FOR INTEGRATED CIRCUITS,” Ser. No. 08/903,453; “TRANSISTOR WITH VARIABLE ELECTRON AFFINITY GATE AND METHODS OF FABRICATION AND USE,” Ser. No. 08/903,452, now abandoned; “SILICON CARBIDE GATE TRANSISTOR AND FABRICATION PROCESS,” Ser. No. 08/903,486, now issued as U.S. Pat. No. 6,936,849; and “TRANSISTOR WITH SILICON OXYCARBIDE GATE AND METHODS OF FABRICATION AND USE,” Ser. No. 08/902,132, now issued as U.S. Pat. No. 5,886,368; each of which was filed on Jul. 29, 1997, and each of which disclosure is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to integrated circuit technology, including dynamic random access memories (DRAMs) and electrically erasable and programmable read only memories (EEPROMS), and particularly, but not by way of limitation, to a floating gate transistor memory that is dynamically electrically alterable and programmable.

BACKGROUND OF THE INVENTION

Dynamic random access memories (DRAMs) are data storage devices that store data as charge on a storage capacitor. A DRAM typically includes an array of memory cells. Each memory cell includes a storage capacitor and an access transistor for transferring charge to and from the storage capacitor. Each memory cell is addressed by a word line and accessed by a bit line. The word line controls the access transistor such that the access transistor controllably couples and decouples the storage capacitor to and from the bit line for writing and reading data to and from the memory cell.
The storage capacitor must have a capacitance that is large enough to retain a charge sufficient to withstand the effects of parasitic capacitances, noise due to circuit operation, and access transistor reverse-bias junction leakage currents between periodic data refreshes. Such effects can result in erroneous data. Obtaining a large capacitance typically requires a storage capacitor having a large area. However, a major goal in DRAM design is to minimize the area of a DRAM memory cell to allow cells to be more densely packed on an integrated circuit die so that more data can be stored on smaller integrated circuits.
In achieving the goal of increasing DRAM array capacity by increasing cell density, the sufficient capacitance levels of the DRAM storage capacitors must be maintained. A “stacked storage cell” design can increase the cell density to some degree. In this technique, two or more capacitor conductive plate layers, such as polycrystalline silicon (polysilicon or poly), are deposited over a memory cell access transistor on a semiconductor wafer. A high dielectric constant material is sandwiched between these capacitor plate layers. Such a capacitor structure is known as a stacked capacitor cell (STC) because the storage capacitor plates are stacked on top of the access transistor. However, formation of stacked capacitors typically requires complicated process steps. Stacked capacitors also typically increase topographical features of the integrated circuit die, making subsequent lithography and processing, such as for interconnection formation, more difficult. Alternatively, storage capacitors can be formed in deep trenches in the semiconductor substrate, but such trench storage capacitors also require additional process complexity. There is a need in the art to further increase memory storage density without adding process complexity or additional topography.
Electrically erasable and programmable read only memories (EEPROMs) provide nonvolatile data storage. EEPROM memory cells typically use field-effect transistors (FETs) having an electrically isolated (floating) gate that affects conduction between source and drain regions of the FET. A gate dielectric is interposed between the floating gate and an underlying channel region between source and drain regions. A control gate is provided adjacent to the floating gate, separated therefrom by an intergate dielectric.
In such memory cells, data is represented by charge stored on the polysilicon floating gates, such as by hot electron injection or Fowler-Nordheim tunneling during a write operation. Fowler-Nordheim tunneling is typically used to remove charge from the polysilicon floating gate during an erase operation. However, the relatively large electron affinity of the polysilicon floating gate presents a relatively large tunneling barrier energy at its interface with the underlying gate dielectric. The large tunneling barrier energy provides longer data retention times than realistically needed. For example, a data charge retention time at 85 □ C is estimated to be in millions of years for some floating gate memory devices. The large tunneling barrier energy also increases the voltages and time needed to store and remove charge to and from the polysilicon floating gate. “Flash” EEPROMs, which have an architecture that allows the simultaneous erasure of many floating gate transistor memory cells, require even longer erasure times to accomplish this simultaneous erasure. The large erasure voltages needed can result in hole injection into the gate dielectric. This can cause erratic overerasure, damage to the gate dielectric, and introduction of trapping states in the gate dielectric. The high electric fields that result from the large erasure voltages can also result in reliability problems, leading to device failure. There is a need in the art to obtain floating gate transistors that allow the use of lower programming and erasure voltages and shorter programming and erasure times.

SUMMARY OF THE INVENTION

The present invention includes a memory cell that allows the use of lower programming and erasure voltages and shorter programming and erasure times by providing a storage electrode for storing charge and providing an adjacent amorphous silicon carbide (a-SiC) insulator.
In one embodiment, the memory cell includes a floating gate transistor, having a reduced barrier energy between the floating gate and an amorphous silicon carbide (a-SiC) insulator. A refresh circuit allows dynamic refreshing of charge stored on the floating gate. The barrier energy can be lowered to a desired value by selecting the appropriate material composition of the a-SiC insulator. As a result, lower programming and erasure voltages and shorter programming and erasure times are obtained.
Another aspect of the present invention provides a method of using a floating gate transistor having a reduced barrier energy between a floating gate electrode and an adjacent a-SiC insulator. Data is stored by changing the charge of the floating gate. Data is refreshed based on a data charge retention time established by the barrier energy. Data is read by detecting a conductance between a source and a drain. The large transconductance gain of the memory cell of the present invention provides a more easily detected signal and reduces the required data storage capacitance value and memory cell size when compared to a conventional dynamic random access memory (DRAM) cell.
The present invention also includes a method of forming a floating gate transistor. Source and drain regions are formed. An a-SiC gate insulator is formed. A floating gate is formed, such that the floating gate is isolated from conductors and semiconductors. The a-SiC gate insulator provides a relatively short data charge retention time, but advantageously provides a shorter write/programming and erase times, making operation of the present memory speed competitive with a DRAM.
The present invention also includes a memory device that is capable of providing short programming and erase times, low programming and erase voltages, and lower electric fields in the memory cell for improved reliability. The memory device includes a refresh circuit and a plurality of memory cells. Each memory cell includes a transistor. Each transistor includes a source region, a drain region, a channel region between the source and drain regions, and a floating gate that is separated from the channel region by an a-SiC gate insulator. The transistor also includes a control gate located adjacent to the floating gate and separated therefrom by an intergate dielectric. The memory device includes flash electrically erasable and programmable read only memory (EEPROM), dynamic random access memory (DRAM), and dynamically electrically alterable and programmable read only memory (DEAPROM) embodiments.
The memory cell of the present invention provides a reduced barrier energy, large transconductance gain, an easily detected signal, and reduces the required data storage capacitance value and memory cell size. The lower barrier energy increases tunneling current and also advantageously reduces the voltage required for writing and erasing the floating gate transistor memory cells. For example, conventional polysilicon floating gate transistors typically require complicated and noisy on-chip charge pump circuits to generate the large erasure voltage, which typically far exceeds other voltages required on the integrated circuit. The present invention allows the use of lower erasure voltages that are more easily provided by simpler on-chip circuits. Reducing the erasure voltage also lowers the electric fields, minimizing reliability problems that can lead to device failure, and better accommodating downward scaling of device dimensions. Alternatively, the thickness of the gate insulator can be increased from the typical thickness of a silicon dioxide gate insulator to improve reliability or simplify processing, since the lower barrier energy allows easier transport of charge across the gate insulator by Fowler-Nordheim tunneling.
According to another aspect of the invention, the shorter retention time of data charges on the floating electrode, resulting from the smaller barrier energy, is accommodated by refreshing the data charges on the floating electrode. By decreasing the data charge retention time and periodically refreshing the data, the write and erase operations can be several orders of magnitude faster such that the present memory is speed competitive with a DRAM. In this respect, the memory operates similar to a memory cell in DRAM, but avoids the process complexity, additional space needed, and other limitations of forming stacked or trench DRAM capacitors.
The memory cell of the present invention can be made smaller than a conventional DRAM memory cell. Moreover, because the storage capacitor of the present invention is integrally formed as part of the transistor, rather than requiring complex and costly non-CMOS stacked and trench capacitor process steps, the memory of the present invention should be cheaper to fabricate than DRAM memory cells, and should more easily scale downward as CMOS technology advances.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like numerals describe substantially similar components throughout the several views.
FIG. 1 is a simplified schematic/block diagram illustrating generally one embodiment of a memory including reduced barrier energy floating electrode memory cells.
FIG. 2 is a cross-sectional view that illustrates generally a floating gate transistor embodiment of a memory cell provided by the present invention.
FIG. 3 is an energy band diagram that illustrates generally conduction band energy levels in a floating gate transistor provided by the present invention.
FIG. 4 is a graph comparing barrier energy vs. tunneling distance for a conventional floating gate transistor and one embodiment of a the present invention having a lower barrier energy.
FIG. 5 is a graph that illustrates generally the relationship between Fowler-Nordheim tunneling current density vs. the barrier energy Φ_GIat various parameterized values E₁<E₂<E₃of an electric field.
FIG. 6 illustrates generally how the barrier energy affects the time needed to perform write and erase operations by Fowler-Nordheim tunneling for a particular voltage.
FIG. 7 is a graph that illustrates generally charge density vs. write/erase time for three different embodiments of a floating gate FET.
FIG. 8 is a cross-sectional view, similar to FIG. 2, but having a larger area control gate-floating gate capacitor than the floating gate-substrate capacitor.
FIG. 9A is a schematic diagram, labeled prior art, that illustrates generally a conventional DRAM memory cell.
FIG. 9B is a schematic diagram that illustrates generally one embodiment of a floating gate FET memory cell according to the present invention.

DETAILED DESCRIPTION OF THE 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 semiconductor-based structure having an exposed surface with which to form the integrated circuit structure of the invention. Wafer and substrate are used interchangeably 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 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.
The present invention discloses a dynamic electrically alterable programmable read only memory (DEAPROM) cell. The memory cell has a floating electrode, which is defined as an electrode that is “electrically isolated” from conductors and semiconductors by an insulator such that charge storage upon and removal from the floating electrode depends upon charge conduction through the insulator. In one embodiment, described below, the floating electrode is a floating gate electrode in a floating gate field-effect transistor, such as used in flash electrically erasable and programmable read only memories (EEPROMs). However, a capacitor or any other structure having a floating electrode and adjacent insulator could also be used according to the techniques of the present invention described below. According to one aspect of the present invention, a barrier energy between the floating electrode and the insulator is lower than the barrier energy between polycrystalline silicon (polysilicon) and silicon dioxide (SiO₂), which is approximately 3.3 eV. According to another aspect of the present invention, the shorter retention time of data charges on the floating electrode, resulting from the smaller barrier energy, is accommodated by refreshing the data charges on the floating electrode. In this respect, the memory operates similar to a memory cell in a dynamic random access memory (DRAM). These and other aspects of the present invention are described in more detail below.
FIG. 1 is a simplified schematic/block diagram illustrating generally one embodiment of a memory 100 according to one aspect of the present invention, in which reduced barrier energy floating electrode memory cells are incorporated. Memory 100 is referred to as a dynamic electrically alterable programmable read only memory (DEAPROM) in this application, but it is understood that memory 100 possesses certain characteristics that are similar to DRAMs and flash EEPROMs, as explained below. For a general description of how a flash EEPROM operates, see B. Dipert et al., “Flash Memory Goes Mainstream,” IEEE Spectrum, pp. 48-52 (October 1993), which is incorporated herein by reference. Memory 100 includes a memory array 105 of multiple memory cells 110. Row decoder 115 and column decoder 120 decode addresses provided on address lines 125 to access the addressed memory cells in memory array 105. Command and control circuitry 130 controls the operation of memory 100 in response to control signals received on control lines 135 from a processor 140 or other memory controller during read, write, refresh, and erase operations. Command and control circuitry 130 includes a refresh circuit for periodically refreshing the data stored on floating gate transistor or other floating electrode memory cells 110. Voltage control 150 provides appropriate voltages to the memory cells during read, write, refresh, and erase operations. Memory 100, as illustrated in FIG. 1, has been simplified for the purpose of illustrating the present invention and is not intended to be a complete description. Only the substantial differences between DEAPROM memory 100 and conventional DRAM and flash EEPROM memories are discussed below.
FIG. 2 is a cross-sectional view that illustrates generally, by way of example, but not by way of limitation, one floating gate transistor embodiment of a memory cell 110. Other structural arrangements of floating gate transistors are included within the present invention. Also included are any memory cells that incorporate a floating electrode (such as a floating electrode capacitor) having, at an interface between the floating electrode an adjacent insulator, a barrier energy that is less than the barrier energy at a polysilicon-SiO₂interface. In the embodiment of FIG. 2, memory cell 110 includes a floating gate FET 200, which is illustrated as an n-channel FET, but understood to include a p-channel FET embodiment as well.
FET 200 includes a source 205, a drain 210, a floating gate 215 electrode, and a control gate 220 electrode. A gate insulator 225 is interposed between floating gate 215 and substrate 230. An intergate insulator 235 is interposed between floating gate 215 and control gate 220. In one embodiment, substrate 230 is a bulk semiconductor, such as silicon. In another embodiment, substrate 230 includes a thin semiconductor surface layer formed on an underlying insulating portion, such as in a semiconductor-on-insulator (SOI) or other thin film transistor technology. Source 205 and drain 210 are formed by conventional complementary metal-oxide-semiconductor (CMOS) processing techniques. Source 205 and drain 210 are separated by a predetermined length for forming an inversion channel 240 therebetween.
FIG. 3 is an energy band diagram that illustrates generally the conduction band energy levels in floating gate 215, gate insulator 225, and substrate 230. Electron affinities χ₂₁₅, χ₂₂₅, and χ₂₃₀describe floating gate 215, gate insulator 225, and substrate 230, respectively, when measured with respect to a vacuum level 300. A barrier energy Φ_GI, which describes the barrier energy at the interface between floating gate 215 and gate insulator 225, is given by a difference in electron affinities, as illustrated in Equation 1.
Φ_GI=χ₂₁₅−χ₂₂₅ (1)
A barrier energy Φ_SG, which describes the barrier energy at the interface between substrate 230 and gate insulator 225, is given by a difference in electron affinities, as illustrated in Equation 2.
Φ_SG=χ₂₃₀−χ₂₂₅ (2)
Silicon (monocrystalline or polycrystalline Si) has an electron affinity χ₂₁₅≈4.2 eV. Silicon dioxide (SiO₂) has an electron affinity, χ₂₂₅, of about 0.9 eV. The resulting barrier energy at a conventional Si—SiO₂interface between a floating gate and a gate insulator is approximately equal to 3.3 eV. One aspect of the present invention provides a barrier energy Φ_GIthat is less than the 3.3 eV barrier energy of a conventional Si—SiO₂interface.
According to one aspect of the invention, the interface between floating gate 215 and gate insulator 225 provides a smaller barrier energy Φ_GIthan the 3.3 eV barrier energy at an interface between polysilicon and silicon dioxide, such as by an appropriate selection of the material composition of one or both of floating gate 215 and gate insulator 225. In one embodiment, the smaller barrier energy Φ_GIis obtained by forming floating gate 215 from a material having a smaller electron affinity χ₂₁₅than polysilicon. In one embodiment, for example, polycrystalline or microcrystalline silicon carbide (SiC) is used as the material for forming floating gate 215. In another embodiment, the smaller barrier energy Φ_GIis obtained by forming gate insulator 225 from a material having a higher electron affinity χ₂₂₅than SiO₂. In one embodiment, for example, amorphous SiC is used as the material for forming gate insulator 225. In yet another embodiment, the smaller barrier energy Φ_GIis obtained by a combination of forming floating gate 215 from a material having a smaller electron affinity χ₂₁₅than polysilicon and also forming gate insulator 225 from a material having a higher electron affinity χ₂₂₅than SiO₂.
The smaller barrier energy Φ_GIprovides current conduction across gate insulator 225 that is easier than for a polysilicon-SiO₂interface. The present invention includes any mechanism of providing such easier current conduction across gate insulator 225, including, but not limited to “hot” electron injection, thermionic emission, Schottky emission, Frenkel-Poole emission, and Fowler-Nordheim tunneling. Such techniques for transporting charge carriers across an insulator, such as gate insulator 225, are all enhanced by providing a smaller barrier energy Φ_GIaccording to the techniques of the present invention. These techniques allow increased current conduction, current conduction at lower voltages across gate insulator 225 and lower electric fields in gate insulator 225, shorter data write and erase times, use of a thicker and more reliable gate insulator 225, and other advantages explained below.
FIG. 4 is a graph illustrating generally barrier energy versus tunneling distance for a conventional polysilicon-SiO₂interface having a 3.3 eV barrier energy. FIG. 4 also illustrates barrier energy versus tunneling distance for an interface according to the present invention that has a barrier energy of Φ_GI≈1.08 eV, which is selected as an illustrative example, and not by way of limitation. The smaller barrier energy Φ_GIreduces the energy to which the electrons must be excited to be stored on or removed from the floating gate 215, such as by thermal emission over the barrier. The smaller barrier energy Φ_GIalso reduces the distance that electrons have to traverse, such as by Fowler-Nordheim tunneling, to be stored upon or removed from floating gate 215. In FIG. 4, “do” represents the tunneling distance of a conventional floating gate transistor due to the 3.3 eV barrier energy represented by the dashed line “OLD”. The tunneling distance “dn” corresponds to a floating gate transistor according to the present invention and its smaller barrier energy, such as Φ_GI≈1.08 eV, for example, represented by the dashed line “NEW”. Even a small reduction in the tunneling distance results in a large increase in the tunneling probability, as described below, because the tunneling probability is an exponential function of the reciprocal of the tunneling distance.
The Fowler-Nordheim tunneling current density in gate insulator 225, which is illustrated approximately by Equation 3 below, is described in a textbook by S. M. Sze, “Physics of Semiconductor Devices,” John Wiley & Sons, New York (1969), p. 496. $\begin{matrix} J = {AE}^{2} ⅇ^{(- \frac{B}{B})} & (3) \end{matrix}$
In Equation 3, J is the current density in units of amperes/cm², E is the electric field in gate insulator 225 in units of volts/cm and A and B are constants, which are particular to the material of gate insulator 225, that depend on the effective electron mass in the gate insulator 225 material and on the barrier energy Φ_GI. The constants A and B scale with the barrier energy Φ_GI, as illustrated approximately by Equations 4 and 5, which are disclosed in S. R. Pollack et al., “Electron Transport Through Insulating Thin Films,” Applied Solid State Science, Vol. 1, Academic Press, New York, (1969), p. 354. $\begin{matrix} A α (\frac{1}{Φ_{GI}}) & (4) \\ B {α (Φ_{GI})}^{\frac{3}{2}} & (5) \end{matrix}$
For a conventional floating gate FET having a 3.3 eV barrier energy at the interface between the polysilicon floating gate and the SiO₂gate insulator, A=5.5×10⁻¹⁶amperes/Volt²and B=7.07×10⁷Volts/cm, as disclosed in D. A. Baglee, “Characteristics and Reliability of 100 Å Oxides,” Proc. 22nd Reliability Symposium, (1984), p. 152. One aspect of the present invention includes selecting a smaller barrier energy Φ_GIsuch as, by way of example, but not by way of limitation, Φ_GI≈≈1.08 eV. The constants A and B for Φ_GI≈1.08 eV can be extrapolated from the constants A and B for the 3.3 eV polysilicon-SiO₂barrier energy using Equations 4 and 5. The barrier energy Φ_GI≈1.08 eV yields the resulting constants A=1.76×10⁻¹⁵amperes/Volt²and B=1.24×10⁷Volts/cm.
FIG. 5 is a graph that illustrates generally the relationship between Fowler-Nordheim tunneling current density vs. the barrier energy Φ_GI, such as at various parameterized values E₁<E₂<E₃of an electric field in gate insulator 225. The tunneling current density increases as electric field is increased. The tunneling current also increases by orders of magnitude as the barrier energy Φ_GIis decreased, such as by selecting the materials for floating gate 215 and gate insulator 225 or otherwise reducing the barrier energy Φ_GIaccording to the techniques of the present invention. In particular, FIG. 5 illustrates a comparison between tunneling current densities at the 3.3 eV barrier energy of a conventional polysilicon-SiO₂interface and at the illustrative example barrier energy Φ_GI≈1.08 eV for which constants A and B were extrapolated above. Reducing the 3.3 eV barrier energy to Φ_GI≈1.08 eV increases the tunneling current density by several orders of magnitude.
FIG. 6 is a conceptual diagram, using rough order of magnitude estimates, that illustrates generally how the barrier energy affects the time needed to perform write and erase operations by Fowler-Nordheim tunneling for a particular voltage, such as across gate insulator 225. FIG. 6 also illustrates how the barrier energy affects data charge retention time, such as on floating gate 215 at a temperature of 250 degrees Celsius. Both write and erase time 600 and data charge retention time 605 are decreased by orders of magnitude as the barrier energy is decreased, according to the present invention, from the conventional polysilicon-SiO₂interface barrier energy of 3.3 eV to the illustrative example lower barrier energy Φ_GI≈1.08 eV for which constants A and B were extrapolated above.
The lower barrier energy Φ_GIand increased tunneling current advantageously provides faster write and erase times. This is particularly advantageous for “flash” EEPROMs or DEAPROMs in which many floating gate transistor memory cells must be erased simultaneously, requiring a longer time to transport the larger quantity of charge. For a flash EEPROM using a polysilicon floating gate transistor having an underlying SiO₂gate insulator 225, the simultaneous erasure of a block of memory cells requires a time that is on the order of milliseconds. The write and erase time of the floating gate FET 200 is illustrated approximately by Equation 6. $\begin{matrix} t = \int_{0}^{t} ⅆ t = \int_{o}^{Q} (\frac{1}{J_{225} - J_{235}}) ⅆ Q & (6) \end{matrix}$
In Equation 6, t is the write/erase time, J₂₂₅and J₂₃₅are the respective tunneling current densities in gate dielectric 225 and intergate dielectric 235, Q is the charge density in Coulombs/cm²on floating gate 215. Equation 6 is evaluated for a specific voltage on control gate 220 using Equations 7 and 8. $\begin{matrix} E_{225} = \frac{V_{220}}{[d_{225} + d_{235} (\frac{ɛ_{225}}{ɛ_{235}})] - \frac{Q}{[ɛ_{225} + ɛ_{235} (\frac{d_{225}}{d_{235}})]}} & (7) \\ E_{235} = \frac{V_{220}}{[d_{235} + d_{225} (\frac{ɛ_{235}}{ɛ_{225}})] - \frac{Q}{[ɛ_{235} + ɛ_{225} (\frac{d_{235}}{d_{225}})]}} & (8) \end{matrix}$
In Equations 7 and 8, V₂₂₀is the voltage on control gate 220, E₂₂₅and E₂₃₅are the respective electric fields in gate insulator 225 and intergate insulator 235, d₂₂₅and d₂₃₅are the respective thicknesses of gate insulator 225 and intergate insulator 235, and ε₂₂₅and ε₂₃₅are the respective permittivities of gate insulator 225 and intergate insulator 235.
FIG. 7 is a graph that illustrates generally charge density vs. write/erase time for three different embodiments of the floating gate FET 200, each of which have a polysilicon floating gate 215, by way of illustrative example. Line 700 illustrates generally, by way of example, but not by way of limitation, the charge density vs. write/erase time obtained for a floating gate FET 200 having a 100 Å SiO₂gate insulator 225 and a 150 Å SiO₂(or thinner oxynitride equivalent capacitance) intergate insulator 235.
Line 705 is similar to line 700 in all respects except that line 705 illustrates a floating gate FET 200 in which gate insulator 225 comprises a material having a higher electron affinity χ₂₂₅than SiO₂, thereby providing a lower barrier energy Φ_GIat the interface between polysilicon floating gate 215 and gate insulator 225. The increased tunneling current results in shorter write/erase times than those illustrated by line 700.
Line 710 is similar to line 705 in all respects except that line 710 illustrates a floating gate FET 200 in which gate insulator 225 has a lower barrier energy Φ_GIthan for line 705, or intergate insulator 235 has a higher permittivity ε₂₃₅than for line 705, or control gate 220 has a larger area than floating gate 215, such as illustrated by way of example by the floating gate FET 800 in the cross-sectional view of FIG. 8. As seen in FIG. 8, the area of a capacitor formed by the control gate 220, the floating gate 215, and the intergate insulator 235 is larger than the area of a capacitor formed by the floating gate 215, the gate insulator 225, and the inversion channel 240 underlying gate insulator 225. Alternatively, or in combination with the techniques illustrated in FIG. 8, the intergate insulator 235 can have a higher permittivity than the permittivity of silicon dioxide.
As illustrated in FIG. 7, the barrier energy Φ_GIcan be selected to reduce the write/erase time. In one embodiment, by way of example, but not by way of limitation, the barrier energy Φ_GIis selected to obtain a write/erase time of less than or equal to 1 second, as illustrated in FIG. 7. In another embodiment, by way of example, but not by way of limitation, the barrier energy Φ_GIis selected to obtain a write/erase time of less than or equal to 1 millisecond, as illustrated in FIG. 7. Other values of write/erase time can also be obtained by selecting the appropriate value of the barrier energy Φ_GI.
The lower barrier energy Φ_GIand increased tunneling current also advantageously reduces the voltage required for writing and erasing the floating gate transistor memory cells 110. For example, conventional polysilicon floating gate transistors typically require complicated and noisy on-chip charge pump circuits to generate the large erasure voltage, which typically far exceeds other voltages required on the integrated circuit. The present invention allows the use of lower erasure voltages that are more easily provided by simpler on-chip circuits. Reducing the erasure voltage also lowers the electric fields, minimizing reliability problems that can lead to device failure, and better accommodating downward scaling of device dimensions. In one embodiment, the barrier energy Φ_GIis selected, as described above, to obtain an erase voltage of less than the 12 Volts required by typical EEPROM memory cells.
Alternatively, the thickness of the gate insulator 225 can be increased from the typical thickness of a silicon dioxide gate insulator to improve reliability or simplify processing, since the lower barrier energy Φ_GIallows easier transport of charge across the gate insulator 225 by Fowler-Nordheim tunneling.
The lower barrier energy Φ_GIalso decreases the data charge retention time of the charge stored on the floating gate 215, such as from increased thermal excitation of stored charge over the lower barrier Φ_GI. However, conventional polysilicon floating gates and adjacent SiO₂insulators (e.g., 90 Å thick) have a data charge retention time estimated in the millions of years at a temperature of 85 degrees C., and estimated in the 1000 hour range even at extremely high temperatures such as 250 degrees C. Since such long data charge retention times are longer than what is realistically needed, a shorter data charge retention time can be accommodated in order to obtain the benefits of the smaller barrier energy Φ_GI. In one embodiment of the present invention, by way of example, but not by way of limitation, the barrier energy Φ_GIis lowered to Φ_GI≈1.08 eV by appropriately selecting the composition of the materials of floating gate 215 and gate insulator 225, as described below. As a result, an estimated data charge retention time of approximately 40 seconds at a high temperature, such as 250 degrees C., is obtained.
According to one aspect of the present invention, the data stored on the DEAPROM floating gate memory cell 110 is periodically refreshed at an interval that is shorter than the data charge retention time. In one embodiment, for example, the data is refreshed every few seconds, such as for an embodiment having a high temperature retention time of approximately 40 seconds for Φ_GI≈1.08 eV. The exact refresh rate can be experimentally determined and tailored to a particular process of fabricating the DEAPROM. By decreasing the data charge retention time and periodically refreshing the data, the write and erase operations can be several orders of magnitude faster, as described above with respect to FIG. 7.
FIGS. 9A and 9B are schematic diagrams that respectively illustrate generally a conventional DRAM memory cell and the present invention's floating gate FET 200 embodiment of memory cell 110. In FIG. 9A, the DRAM memory cell includes an access FET 900 and stacked or trench storage capacitor 905. Data is stored as charge on storage capacitor 905 by providing a control voltage on control line 910 to activate FET 900 for conducting charge. Data line 915 provides a write voltage to conduct charge across FET 900 for storage on storage capacitor 905. Data is read by providing a control voltage on control line 910 to activate FET 900 for conducting charge from storage capacitor 905, thereby incrementally changing a preinitialized voltage on data line 915. The resulting small change in voltage on data line 915 must be amplified by a sense amplifier for detection. Thus, the DRAM memory cell of FIG. 9A inherently provides only a small data signal. The small data signal is difficult to detect.
In FIG. 9B, the DEAPROM memory cell 110 according to the present invention includes floating gate FET 200, having source 205 coupled to a ground voltage or other reference potential. Data is stored as charge on floating gate 215 by providing a control voltage on control line 920 and a write voltage on data line 925 for hot electron injection or Fowler-Nordheim tunneling. This is similar to conventional EEPROM techniques, but advantageously uses the reduced voltages and/or a shorter write time of the present invention.
The DEAPROM memory cell 110 can be smaller than the DRAM memory cell of FIG. 9A, allowing higher density data storage. The leakage of charge from floating gate 215 can be made less than the reverse-bias junction leakage from storage capacitor 905 of the DRAM memory cell by tailoring the barrier energy Φ_GIaccording to the techniques of the present invention. Also, the DEAPROM memory cell advantageously uses the large transconductance gain of the floating gate FET 200. The conventional DRAM memory cell of FIG. 9A provides no such gain; it is read by directly transferring the data charge from storage capacitor 905. By contrast, the DEAPROM memory cell 110 is read by placing a read voltage on control line 920, and detecting the current conducted through FET 200, such as at data line 925. The current conducted through FET 200 changes significantly in the presence or absence of charge stored on floating gate 215. Thus, the present invention advantageously provides an large data signal that is easy to detect, unlike the small data signal provided by the conventional DRAM memory cell of FIG. 9A.
For example, the current for floating gate FET 200 operating in the saturation region can be approximated by Equation 9. $\begin{matrix} I_{DS} = \frac{1}{2} μ C_{0} (\frac{W}{L}) {(V_{G} - V_{T})}^{2} & (9) \end{matrix}$
In Equation 9, I_DSis the current between drain 210 and source 205, C_ois the capacitance per unit area of the gate insulator 225, W/L is the width/length aspect ratio of FET 200, V_Gis the gate voltage applied to control gate 220, and V_Tis the turn-on threshold voltage of FET 200.
For an illustrative example, but not by way of limitation, a minimum-sized FET having W/L=1, can yield a transconductance gain of approximately 71 μA/Volt for a typical process. In this illustrative example, sufficient charge is stored on floating gate 215 to change the effective threshold voltage V_Tby approximately 1.4 Volts, thereby changing the current I_DSby approximately 100 microamperes. This significant change in current can easily be detected, such as by sampling or integrating over a time period of approximately 10 nanoseconds, for example, to obtain a detected data charge signal of 1000 fC. Thus, the DEAPROM memory cell 110 is capable of yielding a detected data charge signal that is approximately an order of magnitude larger than the typical 30 fC to 100 fC data charges typically stored on DRAM stacked or trench capacitors. Since DEAPROM memory cell 110 requires a smaller capacitance value than a conventional DRAM memory cell, DEAPROM memory cell 110 can be made smaller than a conventional DRAM memory cell. Moreover, because the CMOS-compatible DEAPROM storage capacitor is integrally formed as part of the transistor, rather than requiring complex and costly non-CMOS stacked and trench capacitor process steps, the DEAPROM memory of the present invention should be cheaper to fabricate than DRAM memory cells, and should more easily scale downward as CMOS technology advances.

Amorphous SiC Gate Insulator Embodiment

In one embodiment, the present invention provides a DEAPROM having a storage element including a gate insulator 225 that includes an amorphous silicon carbide (a-SiC). For example, one embodiment of a memory storage element having an a-SiC gate insulator 225 is described in Forbes et al. U.S. patent application Ser. No. 08/903,453 entitled CARBURIZED SILICON GATE INSULATORS FOR INTEGRATED CIRCUITS, filed on the same day as the present patent application, and which disclosure is herein incorporated by reference. The a-SiC inclusive gate insulator 225 provides a higher electron affinity χ₂₂₅than the approximately 0.9 eV electron affinity of SiO₂. For example, but not by way of limitation, the a-SiC inclusive gate insulator 225 can provide an electron affinity χ₂₂₅≈3.24 eV.
An a-SiC inclusive gate insulator 225 can also be formed using other techniques. For example, in one embodiment gate insulator 225 includes a hydrogenated a-SiC material synthesized by ion-implantation of C₂H₂into a silicon substrate 230. For example, see G. Comapagnini et al. “Spectroscopic Characterization of Annealed Si_1-xC_xFilms Synthesized by Ion Implantation,” J. of Materials Research, Vol. 11, No. 9, pp. 2269-73, (1996). In another embodiment, gate insulator 225 includes an a-SiC film that is deposited by laser ablation at room temperature using a pulsed laser in an ultrahigh vacuum or nitrogen environment. For example, see A. L. Yee et al. “The Effect of Nitrogen on Pulsed Laser Deposition of Amorphous Silicon Carbide Films: Properties and Structure,” J. Of Materials Research, Vol. 11, No. 8, pp. 1979-86 (1996). In another embodiment, gate insulator 225 includes an a-SiC film that is formed by low-energy ion-beam assisted deposition to minimize structural defects and provide better electrical characteristics in the semiconductor substrate 230. For example, see C. D. Tucker et al. “Ion-beam Assisted Deposition of Nonhydrogenated a-Si:C films,” Canadian J. Of Physics, Vol. 74, No. 3-4, pp. 97-101 (1996). The ion beam can be generated by electron cyclotron resonance from an ultra high purity argon (Ar) plasma.
In another embodiment, gate insulator 225 includes an a-SiC film that is synthesized at low temperature by ion beam sputtering in a reactive gas environment with concurrent ion irradiation. For example, see H. Zhang et al., “Ion-beam Assisted Deposition of Si-Carbide Films,” Thin Solid Films, Vol. 260, No. 1, pp. 32-37 (1995). According to one technique, more than one ion beam, such as an Ar ion beam, are used. A first Ar ion beam is directed at a Si target material to provide a Si flux for forming SiC gate insulator 225. A second Ar ion beam is directed at a graphite target to provide a C flux for forming SiC gate insulator 225. The resulting a-SiC gate insulator 225 is formed by sputtering on substrate 230. In another embodiment, gate insulator 225 includes an SiC film that is deposited on substrate 230 by DC magnetron sputtering at room temperature using a conductive, dense ceramic target. For example, see S. P. Baker et al. “D-C Magnetron Sputtered Silicon Carbide,” Thin Films, Stresses and Mechanical Properties V. Symposium, pp. Xix+901, 227-32 (1995). In another embodiment, gate insulator 225 includes a thin a-Si_1-xC_x:H film that is formed by HF plasma ion sputtering of a fused SiC target in an Ar—H atmosphere. For example, see N. N. Svirkova et al. “Deposition Conditions and Density-of-States Spectrum of a-Si_1-xC_x:H Films Obtained by Sputtering,” Semiconductors, Vol. 28, No. 12, pp. 1164-9 (1994). In another embodiment, radio frequency (RF) sputtering is used to produce a-SiC films. For example, see Y. Suzaki et al. “Quantum Size Effects of a-Si(:H)/a-SiC(:H) Multilayer Films Prepared by RF Sputtering,” J. Of Japan Soc. Of Precision Engineering, Vol. 60, No. 3, pp. 110-18 (1996). Bandgaps of a-Si, a-SiC, a-Si:H, and a-SiC:H have been found to be 1.22 eV, 1.52 eV, 1.87 eV, and 2.2 eV respectively.
In another embodiment, gate insulator 225 is formed by chemical vapor deposition (CVD) and includes an a-SiC material. According to one technique, gate insulator 225 includes a-Si_1-xC_x:H deposited by plasma enhanced chemical vapor deposition (PECVD). For example, see I. Pereyra et al. “Wide Gap a-Si_1-xC_x:H Thin Films Obtained Under Starving Plasma Deposition Conditions,” J. Of Non-crystalline Solids, Vol. 201, No. 1-2, pp. 110-118 (1995). According to another technique, mixed gases of silane and methane can be used to form a-Si_1-xC_x:H gate insulator 225. For example, the source gas can include silane in methane with additional dilution in hydrogen. In another embodiment, gate insulator 225 includes a clean a-Si_1-xC_xmaterial formed by hot-filament assisted CVD. For example, see A. S. Kumbhar et al. “Growth of Clean Amorphous Silicon Carbon Alloy Films By Hot-Filament Assisted Chemical Vapor Deposition Technique,” Appl. Phys. Letters, Vol. 66, No. 14, pp. 1741-3 (1995). In another embodiment, gate insulator 225 includes a-SiC formed on a crystalline Si substrate 230 by inductively coupled plasma CVD, such as at 450 degrees Celsius, which can yield a-SiC rather than epitaxially grown polycrystalline or microcrystalline SiC. The resulting a-SiC inclusive gate insulator 225 can provide an electron affinity χ₂₂₅≈3.24 eV, which is significantly larger than the 0.9 eV electron affinity obtainable from a conventional SiO₂gate insulator. For example, see J. H. Thomas et al. “Plasma Etching and Surface Analysis of a-SiC:H Films Deposited by Low Temperature Plasma Enhanced Vapor Deposition,” Gas-phase and Surface Chemistry in Electronic Materials Processing Symposium, Materials Research Soc., pp. Xv+556, 445-50 (1994).
Gate insulator 225 can be etched by RF plasma etching using CF₄O₂in SF₆O₂. Self-aligned source 205 and drain 210 can then be formed using conventional techniques for forming a FET 200 having a floating (electrically isolated) gate 215, or in an alternate embodiment, an electrically interconnected (driven) gate.

CONCLUSION

The present invention provides a DEAPROM cell. The memory cell has a floating electrode, such as a floating gate electrode in a floating gate field-effect transistor. According to one aspect of the invention, a barrier energy between the floating electrode and the insulator is lower than the barrier energy between polysilicon and SiO₂, which is approximately 3.3 eV, by using an amorphous silicon carbide (a-SiC) gate insulator adjacent to the floating gate. The memory cell also provides large transconductance gain, which provides a more easily detected signal and reduces the required data storage capacitance value.
According to another aspect of the invention, the shorter retention time of data charges on the floating electrode, resulting from the smaller barrier energy, is accommodated by refreshing the data charges on the floating electrode. By decreasing the data charge retention time and periodically refreshing the data, the write and erase operations can be several orders of magnitude faster. In this respect, the memory operates similar to a memory cell in DRAM, but avoids the process complexity, additional space needed, and other limitations of forming stacked or trench DRAM capacitors.
Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art will appreciate that the above-described embodiments can be used in combination, and 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. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.

Claims

1. A method of forming a memory cell comprising:

forming a source region in a substrate;

forming a drain region in the substrate, a channel region being between the source region and the drain region in the substrate;

forming a floating gate; and

forming an amorphous silicon carbide (a-SiC) insulator between the floating gate and the channel region in the substrate, the amorphous silicon carbide (a-SiC) insulator being in contact with both the floating gate and the substrate.

2. The method of claim 1 wherein forming a floating gate and forming an amorphous silicon carbide (a-SiC) insulator further comprises forming at least one of the floating gate and the a-SiC insulator to have an electron affinity such that a barrier energy, defined as a difference between an electron affinity of the floating gate and an electron affinity of the a-SiC insulator, is less than approximately 3.3 eV.

3. The method of claim 2 wherein forming at least one of the floating gate and the a-SiC insulator further comprises forming the floating gate and the a-SiC insulator such that the barrier energy facilitates a data charge retention time on the floating gate of less than or equal to approximately 40 seconds at 250 degrees Celsius.

4. The method of claim 2 wherein forming at least one of the floating gate and the a-SiC insulator further comprises forming the floating gate and the a-SiC insulator such that the barrier energy facilitates a floating gate erase time of less than or equal to approximately 1 second.

5. The method of claim 2 wherein forming at least one of the floating gate and the a-SiC insulator further comprises forming the floating gate and the a-SiC insulator such that the barrier energy facilitates a floating gate erase voltage of less than approximately 12 Volts.

6. The method of claim 1 wherein forming an amorphous silicon carbide (a-SiC) insulator further comprises forming the a-SiC insulator to have an electron affinity that is less than an electron affinity of silicon dioxide (SiO2).

7. The method of claim 1 wherein forming a floating gate further comprises forming the floating gate to have a smaller electron affinity than polycrystalline silicon.

8. The method of claim 1 wherein forming a floating gate and forming an amorphous silicon carbide (a-SiC) insulator further comprises forming at least one of the floating gate and the a-SiC insulator to have an electron affinity such that a barrier energy, defined as a difference between an electron affinity of the floating gate and an electron affinity of the a-SiC insulator, is less than approximately 2.0 eV.

9. The method of claim 1 wherein forming a floating gate further comprises forming the floating gate such that the floating gate is isolated from conductors and semiconductors.

10. The method of claim 1 wherein:

forming a floating gate further comprises forming the floating gate such that the floating gate is transconductively capacitively coupled to the channel region; and

forming a source region further comprises forming the source region in a silicon substrate.

11. A method for forming a transistor comprising:

forming a source region in a silicon substrate;

forming a drain region in the substrate;

forming a channel region in the substrate between the source region and the drain region; and

forming a floating gate separated from the channel region by an amorphous silicon carbide (a-SiC) insulator, the amorphous silicon carbide (a-SiC) insulator being in contact with both the substrate and the floating gate.

12. The method of claim 11 wherein forming a floating gate further comprises forming at least one of the floating gate and the a-SiC insulator to have an electron affinity such that a barrier energy, defined as a difference between an electron affinity of the floating gate and an electron affinity of the a-SiC insulator, is less than approximately 3.3 eV.

13. The method of claim 11 wherein forming a floating gate further comprises forming at least one of the floating gate and the a-SiC insulator to have an electron affinity such that the barrier energy provides a data charge retention time of the transistor that is adapted for dynamic refreshing of charge stored on the floating gate.

14. The method of claim 11, further comprising forming a control electrode opposite from the floating gate and separated from the floating gate by an intergate insulator.

15. The method of claim 14 wherein forming a control electrode further comprises forming the control electrode with a shape such that an area of a capacitor formed by the control electrode, the floating gate, and the intergate insulator is larger than an area of a capacitor formed by the floating gate, the a-SiC insulator, and the channel region.

16. The method of claim 14 wherein forming a control electrode further comprises forming the intergate insulator to have a permittivity that is higher than a permittivity of silicon dioxide.

17. A method of forming a floating gate transistor, comprising:

forming source and drain regions in a silicon substrate;

forming an amorphous silicon carbide (a-SiC) gate insulator on the substrate over a channel region located between the source and drain regions in the substrate; and

forming a floating gate on the a-SiC gate insulator, wherein forming an amorphous silicon carbide (a-SiC) gate insulator and forming a floating gate further comprises forming the a-SiC gate insulator and forming the floating gate such that a barrier energy, defined as a difference between an electron affinity of the floating gate and an electron affinity of the a-SiC gate insulator, is less than approximately 3.3 eV.

18. The method of claim 17 wherein forming an amorphous silicon carbide (a-SiC) gate insulator further comprises forming an amorphous silicon carbide (a-SiC) gate insulator by ion-implantation of C2H2 into a silicon substrate.

19. The method of claim 17, wherein forming an amorphous silicon carbide (a-SiC) gate insulator includes forming an amorphous silicon carbide (a-SiC) gate insulator by laser ablation at room temperature using a pulsed laser in an ultrahigh vacuum or nitrogen environment.

20. The method of claim 17, wherein forming an amorphous silicon carbide (a-SiC) gate insulator includes forming an amorphous silicon carbide (a-SiC) gate insulator by low-energy ion-beam assisted deposition in order to minimize structural defects and provide better electrical characteristics in the substrate.

21. The method of claim 17, wherein forming an amorphous silicon carbide (a-SiC) gate insulator includes forming an amorphous silicon carbide (a-SiC) gate insulator by ion beam sputtering in a reactive gas environment with concurrent ion irradiation.

22. The method of claim 17, wherein forming an amorphous silicon carbide (a-SiC) gate insulator includes forming an amorphous silicon carbide (a-SiC) gate insulator by DC magnetron sputtering at room temperature using a conductive, dense ceramic target.

23. The method of claim 17, wherein forming an amorphous silicon carbide (a-SiC) gate insulator includes forming an amorphous silicon carbide (a-SiC) gate insulator by HF plasma ion sputtering of a fused SiC target in an Ar—H atmosphere.

24. The method of claim 17, wherein forming an amorphous silicon carbide (a-SiC) gate insulator includes forming an amorphous silicon carbide (a-SiC) gate insulator by radio frequency (RF) sputtering.

25. The method of claim 17, wherein forming an amorphous silicon carbide (a-SiC) gate insulator includes forming an amorphous silicon carbide (a-SiC) gate insulator by chemical vapor deposition (CVD).

26. The method of claim 17, wherein forming an amorphous silicon carbide (a-SiC) gate insulator includes forming an amorphous silicon carbide (a-SiC) gate insulator by inductively coupled plasma CVD, such as at 450 degrees Celsius, which can yield a-SiC rather than epitaxially grown polycrystalline or microcrystalline SiC.

27. A method of forming a floating gate transistor comprising:

forming an n-type source region and an n-type drain region in a silicon substrate;

forming an amorphous silicon carbide (a-SiC) gate insulator on the substrate over a channel region in the substrate, the channel region being located between the source region and the drain region; and

forming a polysilicon floating gate on the a-SiC gate insulator.

28. The method of claim 27 wherein:

forming an n-type source region comprises forming an n+-type source region and an n+-type drain region in a silicon substrate;

forming an amorphous silicon carbide (a-SiC) gate insulator further comprises forming the a-SiC gate insulator by ion-implantation of C2H2 into the substrate to have an electron affinity that is less than an electron affinity of silicon dioxide (SiO2);

forming a polysilicon floating gate further comprises forming the a-SiC gate insulator and forming the polysilicon floating gate such that a barrier energy, defined as a difference between an electron affinity of the polysilicon floating gate and an electron affinity of the a-SiC gate insulator, is less than approximately 3.3 eV; and

further comprising:

forming an intergate insulator on the polysilicon floating gate; and

forming a control electrode on the intergate insulator.