Classifications

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  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
  • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
  • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
  • H01L29/76—Unipolar devices, e.g. field effect transistors
  • H01L29/772—Field effect transistors
  • H01L29/78—Field effect transistors with field effect produced by an insulated gate
  • H01L29/786—Thin film transistors, i.e. transistors with a channel being at least partly a thin film
  • H01L29/78651—Silicon transistors
  • H01L29/78654—Monocrystalline silicon transistors
  • H01L29/78657—SOS transistors
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  • H01L21/02104—Forming layers
  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
  • H01L21/02367—Substrates
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  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/02104—Forming layers
  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
  • H01L21/02436—Intermediate layers between substrates and deposited layers
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  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
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  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
  • H01L21/02656—Special treatments
  • H01L21/02664—Aftertreatments
  • H01L21/02694—Controlling the interface between substrate and epitaxial layer, e.g. by ion implantation followed by annealing
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  • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
  • H01L27/02—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers
  • H01L27/12—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being other than a semiconductor body, e.g. an insulating body
• • H—ELECTRICITY
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  • H01L27/02—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers
  • H01L27/12—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being other than a semiconductor body, e.g. an insulating body
  • H01L27/1203—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being other than a semiconductor body, e.g. an insulating body the substrate comprising an insulating body on a semiconductor body, e.g. SOI
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  • H01L28/00—Passive two-terminal components without a potential-jump or surface barrier for integrated circuits; Details thereof; Multistep manufacturing processes therefor
  • H01L28/20—Resistors
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  • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
  • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
  • H01L29/76—Unipolar devices, e.g. field effect transistors
  • H01L29/772—Field effect transistors
  • H01L29/78—Field effect transistors with field effect produced by an insulated gate
  • H01L29/786—Thin film transistors, i.e. transistors with a channel being at least partly a thin film
  • H01L29/78606—Thin film transistors, i.e. transistors with a channel being at least partly a thin film with supplementary region or layer in the thin film or in the insulated bulk substrate supporting it for controlling or increasing the safety of the device
  • H01L29/78618—Thin film transistors, i.e. transistors with a channel being at least partly a thin film with supplementary region or layer in the thin film or in the insulated bulk substrate supporting it for controlling or increasing the safety of the device characterised by the drain or the source properties, e.g. the doping structure, the composition, the sectional shape or the contact structure
  • H01L29/78621—Thin film transistors, i.e. transistors with a channel being at least partly a thin film with supplementary region or layer in the thin film or in the insulated bulk substrate supporting it for controlling or increasing the safety of the device characterised by the drain or the source properties, e.g. the doping structure, the composition, the sectional shape or the contact structure with LDD structure or an extension or an offset region or characterised by the doping profile
• • H—ELECTRICITY
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  • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
  • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
  • H01L29/76—Unipolar devices, e.g. field effect transistors
  • H01L29/772—Field effect transistors
  • H01L29/78—Field effect transistors with field effect produced by an insulated gate
  • H01L29/786—Thin film transistors, i.e. transistors with a channel being at least partly a thin film
  • H01L29/78651—Silicon transistors
  • H01L29/78654—Monocrystalline silicon transistors
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10B—ELECTRONIC MEMORY DEVICES
  • H10B10/00—Static random access memory [SRAM] devices
  • H10B10/15—Static random access memory [SRAM] devices comprising a resistor load element

Definitions

• the field of the invention relates generally to a method for fabricating a semiconductor on insulator composite substrate, such as silicon-on-sapphire (SOS). More particularly, the field of the invention relates to a method for making a resistor, transistor, or memory cell utilizing an ultra thin silicon layer on a sapphire structure and for providing a resistive load which is self-aligned to a corresponding transistor, thereby eliminating a polysilicon layer and providing an extremely compact memory cell or analog circuit.
• SOS silicon-on-sapphire
• fixed charge Dopant atoms and electrically active states (hereinafter called “fixed charge”) in the conduction channel region of MOSFETs are charged and discharged during operation of the device. Since fixed charge is immobile, it does not contribute to
• the "ideal" semiconductor material would thus include a completely monocrystalline, defect-free silicon layer of sufficient thickness to accommodate the fabrication of active devices thereon. Ideal operation of MOSFETs would occur if there were no parasitic charge in the conduction channel.
• threshold voltage is the gate voltage necessary to initiate conduction.
• a common technique for setting threshold voltage is to modify the dopant concentrations in the channel region.
• this approach has the undesirable side effects associated with dopant charge mentioned above.
• adjusting threshold voltage by ion implantation requires at least two and often four masking steps which increase cost and decrease yield.
• a memory cell for a typical computer or microprocessor commonly stores bits of data as a charge on a floating gate.
• a nonvolatile or static random access memory typically stores data as a charge on the gate of a field effect transistor or as the presence or absence of charge on a plurality of cross-coupled pairs of field effect transistors.
• a bit logic state written in a static RAM remains in that same logic state until rewritten to another bit logic state, or until power is turned off. Also, because SRAMs are made from cross coupled active devices, SRAMs are the fastest type of memory. In a dynamic RAM data will disappear, typically, in less than a second unless constantly refreshed.
• SRAMs often are desirable as memory elements because they are fast, do not need refresh clocks, or involve other timing complexities which compete with normal memory access cycles and must be properly synchronized. Thus, for certain systems the SRAM predominates as the memory cell of choice due to its speed and simplicity.
• FETs field effect transistors
• MOSFETs metallic oxide semiconductor field effect transistors
• Cells in conventional SRAMs have used FETs in primarily two configurations: a four MOSFET transistor configuration and a six MOSFET transistor configuration. SRAMs with four transistors in a cell are typically referred to as 4T SRAMs, and SRAMs with six transistors in a cell are typically referred to as 6T SRAMs.
• each cell of a 4T SRAM typically requires two resistors.
• the resistors are formed in a well-known manner to provide a resistance load for a typical MOSFET. To function properly, that is, to enable each memory cell to properly represent logic states, these resistors must be made of a high resistivity material.
• the resistors enable the MOSFET to perform logic switching. That is, the resistors enable the MOSFET to open and close to generate full swings between the power supply voltages. The switching between the power supply voltages represents the two logic states HIGH and LOW. The resistors thereby prevent inadvertent switching at insignificant power levels in a well-known manner.
• Creating this additional layer of material on the semiconductor wafer increases the number and complexity of processing steps required to manufacture an SRAM. These steps can be critical, time consuming and can present significant obstacles in fabricating an SRAM. For example, because of the extra polysilicon layer, the process may have to align separate layers and conductively connect the layers through contact holes or vias. The second layer may have to be connected with a supply voltage in a conventional manner. The steps disadvantageously may require precise lithography tolerances, for example, in order to align the layers. In addition to increasing cost and processing complexity, such steps may dramatically decrease process yield.
• each layer of material increases the maximum step height of the device. This can have the disadvantage of causing a depth of focus problem in the lithography process.
• expensive and complex equipment may be necessary, which substantially increases fabrication cost per chip.
• Polycrystalline silicon is composed of many small (sub-micrometer size) crystals with generally random orientation.
• Conventional methods of ion implantation and subsequent annealing cause polycrystalline crystals to expand thereby reducing the polysilicon's resistivity and compromising its performance as an SRAM resistor. This obstacle can constrain the initial polysilicon deposition and subsequent processing temperature and times of the SRAM.
• a six transistor (6T) SRAM typically includes four NMOS transistors and two PMOS transistors.
• the two PMOS transistors replace the high resistance resistors used in the 4T SRAMs.
• the complimentary n and p channel transistors are adjacent. Accordingly, an additional layer of material (like that used to form the 4T SRAM resistors) ordinarily is not required.
• the 6T SRAM fabrication process can eliminate some of the processing steps used to fabricate a 4T SRAM.
• the 6T SRAM technology also can reduce (by as much as a factor of 1000) the high leakage currents that are commonly found in 4T SRAMs. This is due to the off current of a PMOS device being typically 10 "12 Amps, whereas the resistors typically draw 10 *8 to 10 "9 Amps.
• 6T SRAM The fabrication of a 6T SRAM, however, can present other problems. Because a transistor is more complex than a resistor, it requires more die area. Transistors typically have a larger critical area than the resistors. That is, they present a much larger area for possible defects. This large critical area can reduce manufacturing yield. Because 6T SRAM technology replaces the resistors of the 4T SRAMs with transistors, a memory cell comprising 6T SRAMs typically has a larger critical area than a 4T SRAM cell. Thus, there is a critical need to eliminate defects such as particles, dislocations or the like, which may give rise to leakage current and the formation of parasitic charge. This in turn may facilitate the formation of parasitic bipolar transistor action due to adjacent regions of n and p type material in a transistor of an SRAM memory cell.
• a latch-up condition may occur between adjacent n and p channel transistors.
• This latch-up condition occurs because the substrate typically used in an SRAM does not fully insulate the different regions of the device.
• a "latch-up" conduction occurs when unwanted or parasitic npn and pnp bipolar transistor action causes a low resistance path between the power supply and ground.
• n and p channel transistors must be spaced apart whether they are used in an SRAM control circuitry, or in the core of an SRAM itself. Consequently, the need to isolate adjacent n and p channel transistors used in SRAMs can consume even more cell or die space. Since for a given defect density, large die area results in low yield, the end result can be a low yield, large die and highly expensive SRAM.
• SOS silicon-on-sapphire
• MOSFET microelectronics primarily for applications requiring radiation immunity.
• a silicon film is epitaxially grown on a sapphire substrate.
• the silicon film is thin compared to the source to drain separation (called the channel length) and the insulating substrate is thick enough to ensure no significant electrostatic coupling to a back plane. Due to crystal and thermal expansion mismatches between the silicon and the sapphire, the silicon films are typically heavily populated with crystalline defects and electrically active states. The dominant type of crystalline defects are commonly called "twins".
• the quality of the silicon films can be improved by increasing the thickness of the silicon, hence traditional SOS is made with silicon films between 400 and 800 nanometers thick. This film thickness is capable of supporting transistors with channel lengths down to about 1 micron.
• a composite substrate comprised of a monocrystalline semiconductor layer, such as silicon, epitaxially deposited on a supporting insulative substrate are well recognized. These advantages include the substantial reduction of parasitic capacitance between charged active regions and the substrate and the effective elimination of leakage currents flowing between adjacent active devices. This is accomplished by employing as the substrate an insulative material, such as sapphire (Al 2 O 3 ) and providing that the conduction path of any interdevice leakage current must pass through the substrate.
• An "ideal" silicon-on-sapphire wafer may be defined to include a completely monocrystalline, defect-free silicon layer of sufficient thickness to accommodate the fabrication of active devices therein.
• the silicon layer would be adjacent to a sapphire substrate and would have a minimum of crystal lattice discontinuities at the silicon-sapphire interface.
• Previous attempts to fabricate this "ideal" silicon-on- sapphire (SOS) wafer have been frustrated by a number of significant problems.
• a first significant problem encountered in attempts to fabricate the ideal SOS wafer is the substantial incursion of contaminants into the epitaxially deposited silicon layer.
• substantial concentrations of aluminum contaminants, diffused from the sapphire substrate are found throughout the silicon epitaxial layer.
• the inherent consequence of a high concentration of aluminum contaminants, effectively acting as acceptor-type impurities in the silicon epitaxial layer, is that there are unacceptably high leakage currents between the source and drain regions of p- channel active devices, such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) and MOSFETs (Metal Semiconductor FET). These leakage currents may be of sufficient magnitude that the p-channel active devices may be considered to be always in an "on", or conducting state.
• the SPE process provides a low temperature subprocess for improving the crystallinity of the silicon epitaxial layer of a silicon-on-sapphire composite substrate.
• the SPE process involves the high energy implantation (typically at 40 KeV to 550 KeV) of an ion species, such as silicon, into the silicon epitaxial layer at a sufficient dose (typically
• the thickness of the silicon epitaxial layer is substantially that intended for the completed silicon-on-sapphire composite substrate (typically 3000 A - 6000 A).
• the ion species is implanted through the majority of the epitaxial layer so that the maximum disruption of the silicon crystal lattice is near, but not across, the silicon/sapphire interface to ensure that the amorphous region is adjacent the sapphire substrate. Throughout the ion implantation, the sapphire substrate is kept below about 100°C by cooling with Freon or liquid Nitrogen.
• a single step low temperature (600°C) annealing of the composite substrate is then performed to convert the amorphous silicon layer into crystalline silicon.
• the remaining crystalline surface portion of the silicon layer effectively acts as a nucleation seed so that the regrown portion of the silicon epitaxial layer has a common crystallographic orientation and is substantially free of crystalline defects.
• Lau's SPE process does significantly improve the crystallinity of the silicon epitaxial layer, it also facilitates the diffusion of aluminum from the sapphire substrate (Al 2 O 3 ) into the silicon epitaxial layer, which dopes the silicon film p-type.
• the contaminant concentration resulting from the use of the SPE process is, unfortunately, sufficient to preclude the practical use of integrated circuits fabricated on composite substrates, such as silicon on sapphire, processed with this SPE subprocess.
• the reasons for the failure of active devices to operate correctly are essentially the same as given above with regard to composite substrates fabricated utilizing high temperature processing steps. Additionally, it has been observed that the method described by Lau et al., can leave enough electrically active states in the silicon epitaxial layer to preclude its use for fabrication of integrated circuits using silicon on sapphire.
• U.S. Patent No. 4,509,990 entitled "SOLID PHASE EPITAXY AND REGROWTH PROCESS WITH CONTROLLED DEFECT DENSITY PROFILING FOR HETEROEPITAXIAL SEMICONDUCTOR ON INSULATOR COMPOSITE SUBSTRATES", issued to Vasudev, also describes use of ion implantation and solid phase regrowth to prepare a silicon-on-sapphire wafer.
• a method for fabricating a silicon-on-sapphire wafer very similar to that taught by Lau et al. is described with the additional requirement that the implantation energy and the ion dose are constrained such that they are sufficiently low so as not to exceed the damage density threshold of the sapphire substrate.
• the method describes a residual high defect density in the silicon layer near the sapphire substrate.
• Both embodiments utilize a method for controlling the temperature of the rear surface of the sapphire substrate by mounting the substrate on a heat sink with either a thin film of thermal paste or a thin film of silicon positioned intermediate to the rear surface of the substrate and the heat sink to provide a high heat conductivity interface therebetween.
• the temperature of the heat sink is held at a constant temperature (typically between -20 °C and 250 °C) resulting in a substantial thermal gradient between the rear surface of the sapphire and the silicon layer (typically 150°C to 200 °C).
• the silicon layer is at a temperature falling in the range of 130°C to 450°C. It has been found that the process described by Vasudev can result in incomplete and non-uniform removal of crystalline defects and electrically active states from the silicon layer due to non-uniform thermal contact of the rear surface of the sapphire with the heat sink. When the thermal paste is used, any air bubbles in the paste interface can result in the nonuniform control of the silicon layer temperature. Additionally, further processing of the wafer is made more difficult because it is necessary to completely remove all residues of the thermal paste before proceeding.
• U.S. Patent No. 4,659,392 entitled “SELECTIVE AREA DOUBLE EPITAXIAL PROCESS FOR FABRIC ATING SILICON-ON-INSULATOR STRUCTURES FOR USE WITH MOS DEVICES AND INTEGRATED CIRCUITS", issued to Vasudev, describes another method for tailoring defect densities in regions of silicon on insulator wafers. Using this process, the defect structure and dopant concentrations near the interface between the silicon and the insulator are optimized for specific applications. However, such residual defects would make such silicon films inappropriate for construction of fully depleted transistors. Other methods to prepare silicon on sapphire films have been described.
• the transistors and/or resistors of an SRAM could be manufactured in this layer eliminating some of the disadvantages of conventional SRAMs.
• Such a uniform layer if fabricated on a sapphire substrate would advantageously have its conductivity controlled by normal ion implantation due to the uniform definition imposed by the sapphire substrate.
• Such a layer would initially have no dopant atoms or electrically active states.
• Substantially no electrically active states is defined as an areal density of electrically active states which is approximately 3xl0 n cm- 2 to 5xl0 u cm- 2 .
• an aspect of the present invention provides an ultra thin silicon on sapphire film which is used to manufacture a FET and a resistor load for example a four transistor (4T) SRAM.
• the invention is not limited to an application to SRAM but rather encompasses an integral resistor load which can be integrated with analog components or formed as part of an integrated circuit for electrostatic discharge (ESD) circuitry, mixed signal circuitry, or the like.
• the resistor load can be integrally formed from the same silicon island which forms a corresponding transistor. Because the resistor load can be made from and integral with the ultra thin silicon material, it can be automatically self-aligned to the transistor.
• a 4T SRAM with a self- aligned integrated resistor load comprising an insulating substrate, a layer of silicon formed on the insulating substrate wherein the silicon layer is characterized by a thickness of less than approximately 1,100 A or 1 lOnm.
• the 4T SRAM comprises four NMOS transistors and two self-aligned integrated resistor loads, one for each pair of NMOS transistors, fabricated in the same layer of silicon formed on the insulating substrate, wherein the self-aligned resistor loads are integrally formed in the same film as the transistors and thereby require no second layer of deposited material such as polysilicon.
• the ultra thin silicon layer disposed over the sapphire substrate provides improved heat sinking capability, as well as a diffusion barrier which enables the silicon film to be appropriately implanted with dopant materials and annealed by standard semiconductor fabrication processes while reducing point defects, dislocations, or the like.
• the resistor load can be fabricated integrally in the ultra thin silicon film disposed over sapphire material, the resistor load can be relatively uniform in film thickness and resistivity, resulting in precise resistance values.
• the resistor load can be made as small as the lithography limit of the first masking step.
• FIGS. 1 A- IE illustrate steps in the process of converting an epitaxial silicon on sapphire wafer into a substantially pure silicon on sapphire wafer.
• FIGS. 2A-2E illustrate a MOSFET and fabrication process steps used to manufacture the MOSFET in substantially pure silicon on sapphire material. Cross sectional views are shown for both n and p type transistors. These figures show the device and process through the first level of metallization.
• FIGS. 3A-3C illustrate an embodiment of the invention comprising adding to the device and process depicted in FIGS. 2, gate sidewall spacers, lightly doped drains (LDD), and self-aligned suicide (salicide).
• LDD lightly doped drains
• siicide self-aligned suicide
• FIG. 4 A shows a top view of an n-channel MOSFET with an integrated resistor fabricated in ultrathin silicon on sapphire in accordance with an aspect of the present invention.
• FIG. 4B shows a cross sectional view of the "n-channel MOSFET and the integrated resistor shown in FIG. 4A.
• FIG. 4C shows a cross sectional view of a transistor and resistor from a conventional 4T SRAM cell.
• FIG. 4D shows an electrical schematic diagram of a four transistor SRAM cell fabricated in either bulk Si or ultrathin silicon on sapphire in accordance with an aspect of the present invention.
• FIG. 5 shows a cross sectional view of a mix-signal or analog cell which can be modified to include a self-aligned integrated resistor load according to an aspect of the invention as shown in FIG. 4B.
• the self-aligned resistor aspect of the present invention can be implemented with a NMOS polarity transistor and n- type resistor.
• opposite polarity could be chosen for either or both depending upon circuit design.
• a first aspect of the invention is to provide an ultrathin silicon film on an insulating substrate and processes for making such a film.
• the intrinsic silicon contains no dopant atoms or electrically active states, either within the silicon film or at the interface between the silicon and the sapphire. While complete elimination of all charge states and dopant atoms is not feasible, trace amounts are acceptable within tolerances determined by the application. For example, if a threshold voltage is to be set to an accuracy of ⁇ Volts, the total-charge in the silicon film should be less than about ⁇ /C 0Jt , where C ox is the gate oxide capacitance per unit area.
• the total number of fixed charges ⁇ N i.e., dopant charge plus band gap states plus interface states plus fixed charge in the insulators
• ⁇ N should be less than approximately 2x10" cm" 2 , which is typical of most current devices.
• certain applications may require tighter threshold voltage control, thereby requiring that the total allowable fixed charge in the silicon film be less than approximately 3x10 11 cm "2 while other applications may tolerate total allowable fixed charge up to as much as 5x10" cm "2 .
• a 270nm thick intrinsic silicon film 22 is deposited on a sapphire substrate 12 by epitaxial deposition to form a silicon-on-sapphire wafer 11.
• the silicon film 22 contains a concentration of twin defects 14 and electrically active states 16.
• the thickness of the silicon film 22 is controlled during the epitaxial deposition process using standard processes.
• a 185 kev Beam of Si ions 20 is implanted into the silicon film 22 to a dose of approximately 6x10 14 cm “2 , thus creating a subsurface amorphous region 22A and leaving a surface monocrystalline silicon region 22S.
• the energy and dose of the beam of Si ions 20 are selected so that the amorphous region 22 A extends from an interface 18 formed between the sapphire substrate 12 and the Si film 22 up into the Si film 22 to a thickness which is greater than the desired final thickness of Silicon film.
• the amorphous region 22A is approximately
• the amorphous region 22A in the 270nm thick intrinsic silicon film 22 is created by implantation with the Si ion beam having an energy of 185 kev at a dose of 6x10 14 , cm '2 while maintaining the silicon film 22 at a uniform temperature at or below about 0°C. It has been found that this process will uniformly amorphize layer
• Previous cooling techniques include various techniques for placing the sapphire substrate 12 in contact with a cooled heat sink. Contact between the sapphire substrate and the heat sink was accomplished in a variety of ways including the use of a thermal paste layer interposed between the sapphire and the heat sink; depositing a layer of indium on the sapphire to provide more uniform contact with the heat sink; polishing the sapphire surface to improve contact with the heat sink; etc.
• the ion implantation will cause the substrate temperature to rise, thereby increasing the required dose and/or energy required to amorphize layer 22A to a level where aluminum will out diffuse from the sapphire 12 into the silicon 22.
• An aspect of the present invention overcomes these shortcomings by cooling the sapphire with a flow of cooled gas and by adjusting the gas flow and/or temperature of the gas to insure that the silicon layer 22 is maintained at or below a predetermined temperature.
• the substrate 12 is cooled to a temperature which maintains the surface of the silicon film 22 at a temperature preferably lower than about 0°C.
• FIG IB One configuration for accomplishing these objectives is illustrated in FIG IB.
• FIG. IB Shown in FIG. IB is a configuration for maintaining the silicon film 22 at a uniform temperature at or below about 0°C.
• the SOS wafer 11 is positioned on a support structure 17 in a manner which creates a chamber 21 between the sapphire substrate 12 and the support structure 17, for example, by placing an O-ring 19 between the support structure 17 and the SOS wafer 11. Cooled gas is circulated through the chamber 21 to cool the substrate 12. Since the gas has the same thermal contact with all areas of the substrate 12, uniform cooling is assured. Gas enters the chamber 21 through an inlet 23 and exits the chamber through an outlet 25.
• the SOS wafer 11 is subjected to a thermal anneal step at approximately 550°C in an inert atmosphere (e.g. nitrogen) to induce solid phase epitaxial regrowth from the surface of the monocrystalline silicon region 22S downward through the amo ⁇ hous region 22 A to the interface 18.
• an inert atmosphere e.g. nitrogen
• the anneal temperature is increased to approximately 900-950 °C in an inert atmosphere (e.g.
• a silicon dioxide region 30 having a thickness of approximately 360 nm is then grown in the monocrystalline silicon region 22S by converting the ambient gas in the annealing system from nitrogen to an oxidizing ambient (e.g. steam or oxygen).
• the silicon dioxide region 30 is sufficiently thick to consume all the remaining twins 14 and band gap states 16 in the surface region 225
• FIG. IC the silicon dioxide region 30 is sufficiently thick to leave an approximately 110 nm thick region of substantially pure silicon 28 (i.e., containing substantially zero defects and bandgap states) immediately adjacent the sapphire substrate 12.
• the silicon dioxide film 30 is removed (etched) to result in an approximately 110 nm thick substantially pure silicon film 28 on the sapphire substrate 12.
• the twins 14 and the states 16 in the upper portion of the silicon film are removed by forming the silicon dioxide film 30 and etching it away. Removal of the silicon dioxide film 30 may be delayed if it could serve a masking or other pu ⁇ ose.
• the substantially pure silicon film 28 on the sapphire substrate 12 is now suited for MOSFET fabrication.
• all of the MOSFET processing steps are preferably limited to temperatures less than approximately 950°C in order to maintain the purity of the silicon in channel regions. Additionally, all anneals performed in non-oxidizing conditions are performed at temperatures less than approximately 950° C. Such depletion mode FETs also can be used to fabricate logic devices such as SRAMs in accordance with an aspect of the present invention.
• formation of isolated n-type and p-type regions in the silicon layer 28 is accomplished using a well-known process often referred to as
• LOCOS local oxidation of silicon
• formation of isolated n-type and p-type regions with the LOCOS process begins with the deposition of a silicon dioxide layer 36, a silicon nitride layer 32 and a photo-resist layer 33 on top of the silicon layer 28 of the silicon-on-sapphire wafer 11 shown in FIG. IE.
• individual islands (36p,32p,33p) and (36n,32n,33n) of the silicon dioxide layer 36, silicon nitride layer 32 and photo-resist layer 33 are formed on the surface of the silicon layer 28 as shown in FIG. 2B.
• Standard masking and ion implantation techniques are used to form a silicon n-type region 22N and a silicon p-type region 22P.
• the silicon n-type region 22N is formed by ion implantation of the silicon layer 28 underlying the island (36n,32n,33n) with phosphorus and the silicon p-type region 22P is formed by ion implantation of the silicon layer 28 underlying the island (36p, 32p, 33p) with boron.
• the silicon n-type region 22N is isolated from the silicon p-type region 22P by the growth of a silicon dioxide region 34.
• the silicon dioxide regions 34 are grown by introducing the wafer 29 shown in FIG. 2B into a high temperature (less than approximately 950 °C) oxidizing ambient environment.
• the silicon dioxide isolation regions 34 extend down to the sapphire substrate 12.
• FIG. 2C shows regions 22N and 22P fully isolated from each other by the silicon dioxide isolation regions 34 for complementary MOS transistors.
• Alternative isolation techniques may also be employed.
• the silicon layer 28 (FIG. IE) may be etched into individual islands (sometimes called "mesas") .
• the silicon islands 22N and 22P become individual isolated islands or mesas.
• a subsequent stage 41 of the MOSFET process is shown in FIG 2D.
• the n-type and p-type regions 22N and 22P are further processed to form self-aligned sources 42S and 52S, conduction regions 44 and 54, and self- aligned drains 42D and 52D, respectively.
• gate insulators 40 and gate conductive layers 48 and 58 form a control gate structure.
• the control gate structure is formed by thermal oxidation of the gate insulators 40 followed by deposition and patterning of a chosen gate conductive layer 48 for the p-channel and 58 for the n- channel.
• the gate length i.e., the distance separating the source 52S from the drain 52D, be maintained at more than about 5-10 times the thickness of the conduction region.
• a 500 nm gate length should be made in a silicon film thinner than about 100 nm, and preferably closer to 50 nm.
• self-aligned sources and drains 42S, 42D, 52S and 52D are formed by ion implantation or diffusion.
• Doping the source and drain regions of thin silicon films is subject to certain limitations. For example, ion implantation doping can amo ⁇ hize the entire thickness of the source/drain region. An amo ⁇ hized film will not properly recrystallize from the sapphire substrate and high resistivity may result. Therefore, it is preferable that the source and drain regions be formed by diffusion doping since the sapphire substrate forms a diffusion barrier to the dopant atoms.
• Diffusion doping of the source/drain regions represents an improvement over conventional MOSFET designs using implantation doping in that very thin (i.e., shallow) source/drain regions 42S, 42D, 52S and 52D having low resistivities can be fabricated by means of a single diffusion step.
• the sapphire substrate 12 is an effective diffusion barrier and since the depth of the source and drain regions 42S, 42D, 52S and 52D are determined by the thickness of the silicon film, forming shallow source and drain regions is controlled by the structure, not by diffusion time and temperature, as in conventional transistor processing. Therefore diffusion doping can be used for scaled down dimensions. Diffusion doping has several advantages over ion implantation including: the host silicon is not damaged or transformed into amo ⁇ hous regions; the process is inherently scalable to the thinnest silicon films; and higher doping concentrations can be achieved.
• Threshold voltage of the control gate structure is initially determined by correctly choosing the gate conductor material according to its so-called metal work function. If necessary, further adjustments to the threshold voltage are made by introducing appropriate dopant atoms into the conduction channel, for example by ion implantation into the conduction regions 44 and 54. In accordance with the present invention, no dopant atoms other than those introduced for threshold adjustment (or to ensure surface channel conduction, see below) are present in the conduction channel regions 44 and 54.
• MOSFETs in substantially pure silicon on sapphire in accordance with the present invention, only minimal concentrations of dopant atoms (if any) are present, thereby eliminating parasitic charge and its associated degradations discussed above.
• Gate conductor layers 48 and 58 are often multilayer structures. In this case, the threshold voltage is determined by the characteristics of the primary gate conductor layer, i.e., the layer which is immediately adjacent the gate insulator 40.
• Conductive layers above the primary gate conductor layer are included for various reasons, especially to reduce series resistance (See FIG. 3 and discussion below for an example). However, such secondary gate conductive layers do not affect the threshold voltage of transistors.
• Each of the gate materials cited below has various applications when the material is in contact with the gate insulator 40.
• p + and n + polysilicon gate materials used in various combinations in n-type MOSFETS and p-type MOSFETS, are-useful in designing and fabricating digital and analog circuits, voltage reference circuits and memory type circuits.
• p + polygermanium is a good choice for high performance digital logic where symmetric threshold voltages for n- and p-type MOSFETs are desired.
• Any conductive material which has a metal work function at the center of silicon's band gap results in symmetric threshold voltages for n- and p-channel MOSFETs.
• Examples of such materials are tungsten, chrome, indium tin oxide, and titanium nitride, among others.
• the material may be different or the same for each transistor type (regions 48 and 58) depending on the desired threshold voltage.
• V m and V ⁇ are the threshold voltages of n- and p-channel MOSFETs, respectively.
• region 48 could be p + polysilicon and region 58 could be n + polysilicon (i.e., different materials). If threshold voltages of +V2 Volt for the n-channel and - ⁇ _ Volt for the p-channel were desired, regions 48 and 58 could be p + polygermanium, tungsten, indium tin oxide or titanium nitride (i.e., the same material). Numerous other material choices, and therefore other choices of threshold voltages, are also available.
• the gate dielectric material 40 is grown and the gate conducting materials 48 and 58 are deposited using process conditions which avoid introduction of states or fixed charges into the channel regions 44 and 54. Specifically, processing temperatures and ambients are chosen to avoid generation of interface states or fixed charge in the dielectric. Therefore, as previously discussed, processing temperatures should be kept below approximately 950°C. Also, for p + doped conductors as gate material 48 or 58, processing temperatures, times and ambients should be chosen to avoid diffusion of the dopant atoms from the gate conductors 48 and 58 through the gate dielectric insulator 40 into the silicon films 44 and 54. Diffusion barriers such as silicon nitride as part of the gate dielectric insulator 40 can be used to prevent such dopant migration.
• Surface channel transistor behavior occurs when conduction occurs in the silicon channels 44 and 54 at the interface between the gate insulator 40 and the silicon films 44 and 54.
• Such a device is defined herein as an "intrinsic surface channel MOSFET.” Additional dopant atoms, such as boron, phosphorous or arsenic, may be introduced into the channel regions 44 and 54 to further adjust the threshold voltage of the intrinsic surface channel MOSFET.
• FIG. 2E A next stage 51 of the MOSFET fabrication process is shown in FIG. 2E.
• insulating layer 62 and metal layer 64 are deposited and patterned for interconnecting devices as desired. Specifically, an interlevel insulating layer 62 is deposited and patterned, followed by deposition and patterning of a metallic conductor interconnecting layer 64.
• annealing step serves two primary functions: to remove states and charge which may have been introduced during the previous processing steps and to sinter different metallic layers to form low resistance contacts.
• source and drain junctions are deep enough to ensure that no metal will diffuse through them and into an underlying silicon substrate, thereby destroying transistors. In the current invention, such a failure mechanism does not exist since only sapphire 12 is found beneath the source and drain regions 42S, 42D, 52S and 52D.
• a further aspect of the invention comprises a lightly doped drain (LDD) structure or self-aligned suicide (salicide).
• LDD lightly doped drain
• siicide self-aligned suicide
• a sidewall spacer 60 is deposited and etched adjacent to the gate structure comprising the gate insulator 40 and conductor 48, 58.
• final self-aligned sources and drains 42S, 42P, 52S and 52D are formed by ion implantation or diffusion.
• gate conductors 48 and 58 such as polysilicon or polygermanium
• the structure is coated with a metallic material and reacted to form metallic compounds 48M and 58M in the upper portion of gate conductors 48 and 58 as well 42M and 52M in source and drain regions 42S, 42P, 52S and 52D. Stripping unreacted metal from sidewall spacers 60 completes the salicide (or germanide) processing.
• suicide regions 42M, 52M, 48M, and 58M are separated from each other by the sidewall spacers 60.
• the thickness of metallic regions 42M, 48M, 52M and 58M is controlled by the amount of metallic material which is deposited.
• the salicide option exists independently of LDD doping level.
• FIG. 3C a complementary MOS structure is shown with both LDD and salicide options included after metallization as described above for FIG 2.
• the MOSFET can be completed according to processes well known in the art, maintaining temperatures below approximately 950°C.
• a substantially pure (or intrinsic) island of silicon 70 is provided on a sapphire or other insulating substrate.
• the substrate is not shown as it underlies the silicon and is not visible as is well known to those of ordinary skill in the art.
• the process for producing silicon island 70 is described above with reference to Figures 1A-1E.
• the island is patterned by standard photolithography and etch techniques.
• An n-channel MOSFET 71 is then fabricated in the silicon island 70 as described with reference to Figures 2 A through 2C.
• Source region 72 and drain region 76 are preferably formed by ion implantation or by diffusion doping as shown in Fig. 2D.
• the sapphire substrate 75 forms a diffusion barrier to the dopant atoms.
• This advantageously enables new thin source/drain regions with closely controlled resistivity to be formed by means of ion implantation or diffusion. For very thin doped regions, it is difficult to implant without going too deeply.
• region 76 is an n + region approximately 100 nm thick
• ion implantation is actually more effective than diffusion doping. Diffusion doping may be more effective at 50 nm thick films.
• the depth of source and drain regions 72 and 76, respectively is controlled by the structure of the diffusion barrier formed by the interface between the thin silicon layer or island 70 and sapphire substrate 75. This advantageously enables a higher doping concentration to be achieved.
• MOSFET 71 for example, may be an intrinsic surface channel MOSFET which is formed on silicon island 70 as was explained with reference to Figure 2D.
• a single silicon island formed in accordance with the process described with reference to Fig. 2A-2C provides a silicon processing base for
• an n-channel MOSFET 71 comprises a source 72, a gate 74 and drain 76 fabricated in ultrathin silicon on a sapphire substrate in accordance with the processes described with reference to Figure 2D.
• a region 78 of the silicon island 70 extends from MOSFET 71.
• Silicon island 70 is formed by patterning nitride layer 32 of Fig. 2A-2C to result in the geometry shown in Fig. 4 A.
• region 78 comprises a portion of and is integral with the ultra-thin silicon island 70 which makes up the MOSFET 71.
• An integrated resistor load 80 is formed within region 78 of island 70.
• Resistor load 80 is doped sufficiently to make it conductive to the desired degree, or can be left undoped for the highest possible resistivity.
• resistor load 80 may be doped with phosphorus atoms at a concentration of less than 10 16 cm '2 .
• the desired resistance value of the resistor load 80 is typically in the range of 10 8 - 10 9 ohms for SRAMs. For analog components, resistor values in the range of 10-10,000 ohms are used.
• the resistance of resistor load 80 can be closely controlled by an ion implantation process as was set forth in the discussion referencing Figure 2D. Of course, such ion implantation would occur prior to or during processing of MOSFET 71.
• region 80 could be implanted during a threshold implant or LDD implant, or by means of special masking and implant steps prior to the n+ source/drain implant.
• Region 80 is photolithographically protected from the n + implantation processes used to form MOSFET 71 and contact area 82.
• resistor load 80 can be formed by selectively amo ⁇ hizing it throughout its thickness. The process for creating such an amo ⁇ hous region was set forth in the discussion with reference to Figure IB and following. However, in this application the ion implant energy and dose are chosen to amo ⁇ hize the entire film thickness. It is necessary to mask the remainder of the integrated circuit before using the step of Fig. IB to a amo ⁇ hize resistor region 80.
• the amo ⁇ hized region 80 of island 70 is then annealed at a temperature sufficient to convert the amo ⁇ hized portion to polycrystalline silicon.
• Such a thermal anneal step comprises heating the wafer to a temperature above 650°C in an atmosphere of nitrogen gas.
• the thermal anneal to polycrystalline silicon normally occurs as part of the MOSFET source/drain dopant activation step.
• doping techniques as described with reference to Figure 2D may be employed to precisely control the desired resistance of resistor load 80.
• the foregoing doping techniques can be proportioned as used in appropriate combinations to achieve precisely desired load characteristics for resistor load 80. These characteristics include the ability to achieve a desired resistance value, size, temperature coefficient or other parameters for enhanced manufacturability. In general, higher dopant concentrations or anneal temperatures will result in lower resistivity values.
• a heavily doped n + region 82, bounded by W x L c provides a contact to the resistor load 80.
• Heavily doped n + region 82 is also formed by patterning and during the NMOS source/drain implant. The doped region 82 will be contacted by metal during subsequent processing.
• the resistor load 80 can be lightly doped in accordance with techniques which are well known in order to control its polarity.
• inter-level dielectric and associated masking steps advantageously can be eliminated from the process steps which would otherwise be required to integrate a resistor load.
• the ultra thin film silicon on sapphire resistor load 80 is formed in the island region 78, it is self-aligned to an NMOS transistor (such as MOSFET 71) and requires no contact or second layer of polysilicon to form the resistive load. This also eliminates processing steps necessary to form resistor loads in conventional techniques for fabricating a 4T SRAM.
• the resistor load 80 is made of ultrathin silicon material, and disposed over sapphire substrate 75 it is extremely uniform in film thickness and resistivity, resulting in accurate resistance values. Since the silicon film begins as an intrinsic or lightly doped layer, its resistivity can be very high, resulting in compact high value resistors. Finally, because resistor load 80 is disposed on sapphire substrate 75, it has no parasitic capacitance to an underlying depletion region, which would be the case if a diffused resistor were made in bulk silicon. This advantage is described with respect to Figure 2D.
• the sapphire substrate 75 advantageously acts as a diffusion barrier.
• the ultra thin silicon disposed on the sapphire substrate 75 overcomes a principal disadvantage in conventional MOSFET integrated circuits in that the silicon on sapphire layer inhibits the formation of spurious channels or the formation of regenerative, parasitic bipolar transistor action.
• the silicon on sapphire configuration thus acts as a channel stopping measure which can enable MOSFET integrated circuits to be fabricated with greater component density.
• FIG. 4B shows a cross-sectional view of the embodiment of FIG. 4 A.
• the elements shown in FIG. 4B are the same elements in FIG. 4A.
• FIG. 4B demonstrates where n and p type dopants are implanted by the aforedescribed methods into the
• MOSFET 71 MOSFET 71.
• contact openings 73 to the structure are formed at gate 74 and n+ regions 72 (source), 76 (drain) and 82 (contact to resistor load 80) as previously described in FIG. IB.
• resistor load 80 and MOSFET 71 are formed in a single silicon island. Due to the integral formation of resistor load 80 with drain 76, the addition of a dopant to resistor load 80 easily can be controlled to provide overvoltage protection to the MOSFET 71. It will be appreciated that integral resistor load 80 also can be implemented in other overvoltage protection circuits, such as in electrostatic discharge (ESD) circuitry. It also will be appreciated that resistor load 80 of the present invention advantageously can be made as small as the lithography limit of the first masking step. Its size is significantly smaller than both PMOS transistors substituted for resistors in conventional 6T SRAMs and the resistors currently used in 4T SRAMs.
• the sapphire substrate 75 acts as a diffusion barrier.
• the source 72 and drain 76, of MOSFET 71, as well as resistor load 80, are fabricated in an ultra thin layer of silicon disposed on sapphire substrate 75.
• the depth of the source 72 and drain 76 is determined by the thickness of the silicon layer, that is, by structure, rather than by diffusion time and temperature as in conventional transistor processing. This has the advantage of simplifying processing steps and further provides increased control of device parameters especially for very thin regions (less than 100 nm). Accordingly, the depths of the source and drain can be more precisely controlled than was previously possible.
• FIG. 4B shows a cross sectional view of a MOSFET and a resistor from a conventional four transistor SRAM cell.
• a principal disadvantage of the SRAM cell of Figure 4C is the high topography which must be present in the metal layers (metal 1 and 2), and in the layers of polycrystalline silicon (Poly 1 and 2).
• an extra layer of polysilicon (Poly 2) must be used.
• the first polysilicon layer (Poly 1) is characterized by a thickness of approximately 300 nm.
• a layer of CVD oxide (for example, SiO 2 ) is disposed above the first polysilicon layer.
• the SiO 2 layer typically has a thickness of approximately
• a second layer of polycrystalline silicon (Poly 2) is disposed over the CVD oxide layer.
• the Poly 2 layer further must be fabricated with a thickness on the order of 300 nm and a high resistivity to provide sufficient resistance values. The result is a device having disadvantageously high topography, as shown in Figure 4C.
• resistor load 80 of the present invention in a 4T SRAM eliminates the need for a second layer of polysilicon. Because the resistor load 80 can be self-aligned to the transistor drain as shown in Figure 4A and 4B, it is possible to construct a transistor for an SRAM having no additional topography height due to addition of the integrated resistor.
• FIG. 4D illustrates a circuit diagram of an embodiment of a 4T SRAM cell 91.
• This circuit is the same for an SRAM manufactured in bulk Si or in the ultra-thin silicon described herein.
• SRAM 91 comprises four n-channel transistors 94 A 94B, 94C and 94D.
• the resistor loads for the SRAM cell 91 are shown at 80B and 80D.
• Resistor load 80B is coupled between the supply voltage Vcc and the drain of NMOS transistor 94B. Resistor load 80B is also coupled to the gate 74D of NMOS transistor 94D and to the internal node of transistor 94 A.
• NMOS transistor 94B The source of NMOS transistor 94B is connected to ground.
• resistor load 80D is coupled with the drain 76D of NMOS 94D and with the gate
• NMOS transistor 94B The NMOS transistors are cross coupled in a well known manner.
• MOSFETs 94A and 94C act as transmission gates that isolate or connect the logic outputs 108 of the cell with the bit lines 106. These two transistors are turned on and off by word line 104.
• the gates 95A and 95C of MOSFETs 94A and 94C, respectively, are first held positive. This is done by selecting the row with the word (also called row select) line 104. The cell is chosen by also providing power to the bit lines 106 A and 106C. Upon selecting the cell, the drain 76B of MOSFET 94B is connected to bit line 106 A and the drain 76D of MOSFET 94D is connected to bit line 106C. A logic 1 is written to the cell by forcing bit line 106 A to logic 1 and bit line 106C to logic 0. Forcing the bit lines to these levels turns on MOSFET 94D and turns off MOSFET 94B. At this point, turning off the MOSFETs
• the cell can be read by again turning on MOSFETs 94A and 94C. Instead of applying a data bit to the cell through the bit lines, however, the SRAM sense amplifier circuitry (not shown) determines the status of the bit lines to determine what logic level is stored in the cell.
• the SRAM cell 91 fabricated in ultrathin silicon on sapphire technology generates an efficient, low cost device in comparison to a conventional SRAM inco ⁇ orating MOSFET transistors.
• Figure 5 shows a cross sectional view of a conventional mixed signal or analog cell 200 which can be modified by inclusion of the integrated self-aligned resistor load 80 of the present invention.
• a mixed signal or analog cell inco ⁇ orating integrated resistor load 80 can be made with greatly reduced dimensions while at the sate time reducing the number of processing steps.
• analog cell 200 comprises a conventional LOCOS layer 202 comprising a first layer of SiO 2 formed over a silicon substrate (not shown).
• a first transistor 204 has a drain 206, a source 208 and a gate 212.
• a first low resistivity layer of polycrystalline silicon 212 is provided over gate oxide 210 in a well known manner.
• a CVD oxide, for example, SiO 2 layer 214 is provided over the polycrystalline silicon layer 212.
• SiO 2 layer 214 provides insulation between a first metal layer 216 and polysilicon 212.
• Metal interconnect 216A connects drain 206 and resistor 218 made in a second polysilicon layer to ensure higher resistivity than the first polysilicon layer 212.
• a second metal interconnect 216B connects the other side of resistor 218 to another part of the circuit.
• integrated resistor 80 replaces resistor 218 and metal interconnect region 216A.
• Integrated resistor 80 has less topography height and consumes less area.
• integrated resistor 80 also requires fewer processing and lithography steps.
• the resistor load of the present invention also can be self-aligned to another resistor, to a capacitor or even to an inductor or to multiple components.
• the self-aligned resistor load according to this aspect of the present invention would provide the same advantages as previously described.
• the SRAM cell should be seen as only one specific implementation of the invention.
• the invention described herein is not limited to a four transistor SRAM cell, but rather is intended to apply to many other components and equivalent structures, such as analog components which can be fabricated with reduced dimensions using the self-aligned resistor load in accordance with the present invention. Therefore, persons of ordinary skill in this field are to understand that all such equivalent structures are to be included within the scope of the following claims.

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• 1996
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